This site has a compilation of vids that show art evolution from different manga authors. Some are really drastic. (The site itself is NSFW.)
The artwork for the original Slayers novels shifted rather oddly. The first few novels featured characters with very delicately detailed clothing, a trait that fades away in the later novels. Another notable change is the faces of the characters: the female characters have very rounded eyes and cheeks, and the males actually have masculine jaw lines; Gourry the swordsman and Rezo the priest looked far different originally. By the eighth novel, all of the characters had generic pointed chins and faces, and the anime adaptation followed suit with these designs.
Many changes can be attributed to the fact that Araizumi is now using Photoshop and SAI to draw and color his art, resulting in a saturated, pastel-like color palette, whereas, up until around that time, he used all traditional media, followed by it mixed with a minimal amount of technology. Along with this, most of the characters, especially Zelgadis, now resemble their anime designs.
There is also the manga adaptation of the first two anime seasons; the first half of the manga was well-detailed and fluidly drawn; by the 5th graphic novel the art deteriorates severely, as if the authoress lost all interest in doing the work. By the final graphic novel, all of the characters resemble paper cut-outs.
Jiraishin was Tsutomu Takahashi's first foray into manga. In the beginning the art is competently done but somewhat generic. Near the middle of the series one begins to notice Takakashi's growing expertise; the characters begin getting much more stylized and distinctive, and the inking starts to take a life of its own (with an emphasis on deep blacks and thin lines), giving Takahashi his signature "sketchy" look, which was continuously refined in his more recent works like Jiraishin Diablo, Sidooh, Skyhigh, and Hito Hitori Futari. This distinct aesthetic would be passed down to his assistant Tsutomu Nihei and Nihei's own assistant Hayashida Q (creator of Dorohedoro).
The art style of Tsutomu Nihei's Blame! changed not once, not twice, but several times throughout its course. Thankfully, Nihei's style appears to have finally become consistent.
However some of his fans decry the art of The Knights Of Sidonia as being too different from the styles featured in Biomega and Blame!
The characters in the Ah! My Goddess manga went through several different designs before they finally stabilized somewhat, and minor changes are still being made in the latest volumes. This webpage◊ shows just how much Belldandy's look alone evolved.
You can see Ken Akamatsu's style evolve by reading his various series in order. Late AI Love You looks like early Love Hina, late Love Hina looks like early Mahou Sensei Negima!, etc. Mahou Sensei Negima! is getting long enough that there's a marked difference between the first few chapters and the more recent ones. In particular, Negi is drawn noticeably different, although whether this is just Art Evolution or an intentional reflection of the fact that Negi is about two years older is up for debate. The first chapters of Negima had a very different style from Love Hina, a conscious shift that was later unmade when Akamatsu slowly returned to his older, more familiar style (which then continued changing from that).
Ai Yori Aoshi in its first chapters the characters look quite amateurish and not as great, but the characters get much better looking at the series goes on. Then the art takes an inexplicable drop in quality in the last volume or two, carrying into Umi no Misaki before becoming good again after a while.
Nobuyuki Anzai's previous work, Flame of Recca was the exact opposite as well as a great example of this trope, comparing the crude and bland designs of the first volume and the fine and visually appealing art of the later volumes, you'd think he was making a hentai at first.
The change in Godchild is nothing short of drastic. The series began in the 1990s, and then later in 2001 Kaori Yuki returned to it. The art in the earlier volumes is very typical shojo, and does not stand out. Later on, however, the art is exceptionally detailed and the people are rendered much more realistically. Here is an example from the beginning of the series and here's an example from the latter part of the series.
The art style in the Gravitation manga changes so radically from Vol. 1 to 12 that if you pick a random page out of each of the two volumes, you wouldn't believe that they were made by the same artist, much less that the main character is the same in each one. Just compare this image◊ of Shuichi in volume 1 to this one◊ of him in volume 6.
Individual characters in the Genshiken manga start out drawn fairly realistically, but grow more stylized and cartoonish as the series goes on. By the time the ninth volume rolls around, you wonder if this is how the characters in the first volume would have depicted theelves in a doujinshi.
The character designs in Urusei Yatsura slowly changed over the series. This normally wouldn't be very noticeable from episode to episode, however, the Title Sequence with the old character designs didn't change for a large part of the series. The manga, on the other hand, has a quite different style at the beginning before settling into the style more usually associated with Rumiko Takahashi.
The change in character designs for Rumiko Takahashi's other series Ranma ˝ is even more noticeable. Compare the rounder, cartoony character designs of the earlier volumes to the lankier, moreserious ones in later books. And the gradual shift to more stylized figures over the course of the anime, culminating in the "Gainax treatment" that all the girls got for the 2nd movie (especially Nabiki) .
In fact, you can trace that evolution (well, from Urusei to Ranma anyway) all the way rather quickly with Mermaid Saga, which she worked periodically on from 1984 to 1994.
An even better example would be One Pound Gospel, on which she worked from 1988 to 2007. The first 2 volumes look like Urusei Yatsura/Maison Ikkoku, the 3rd one like Ranma ˝ and the 4th like InuYasha.
Ranma's braid seemed to do a reverse evolution, however, going from much more realistic-looking in the earlier volumes to something that looks like three balls and the end of a paintbrush.
JoJo's Bizarre Adventure has come a long way from its early chapters. In Part 1, the characters were so overmuscled that they barely looked human, and moved their joints in such strange ways that they looked even less so. Contrast to Part 5, where everyone looks slim but realistically muscled, and also very feminine.
Here◊ is a timeline of the art change throughout JoJo's Bizarre Adventure. And if you want to go back even further than JoJo, this◊ is a panel from Hirohiko Araki's previous series, Baoh the Visitor.
Hunter × Hunter, on the other hand, did not fare as well. Recent chapters of the manga were drawn while the author was ill, and have been noticeably decreasing in art quality, some of them looking like literal scribbles. The author wisely chose to take a break and focus on recovering from his illness, and the latest chapters he released after the end of his hiatus are of fairly good quality. Thankfully, he also revises the art for the tankoban collections, so it looks much better.
To compare, here◊ is chapter 252 when it was released in Shonen Jump. Yes, the entire chapter (previous ones as well) looked that bad. Here◊ is the version released in the tankoban.
Early volumes of Death Note are done in the fairly typical Shōnen style, with frequent Face Faults, Visible Sighs and Sweat Drops. As the series gets darker though, all these effects disappear and the artwork becomes sharper and more realistic. A flashback to the first chapter uses this for effect as an Art Shift. When Light gives up his memories and reverts to his mostly-innocent personality from the beginning, his face changes dramatically. His eyes open much wider than normal, stress lines disappear and he see younger and less evil. The artist of the series admitted that it was hard to unlearn everything he'd innovated and go back to the original sketches.
The change from the beginning of the Hikaru no Go to the end (5 years) is remarkable. At the start, the art was relatively cartoony, while the series wrapped up with art that was far more realistic and subdued, like the artist's later works of Death Note and Bakuman。.
Bakuman。 loosens up significantly on the realism over the course of the series and indulges in more and more wild facial expressions (especially when Hiramaru is involved.)
Detective Conan is rather interesting, as the anime has actually followed the art evolution of the manga very closely. Given that Gosho Aoyama's art style has evolved considerably in the past 13 years, it makes for a very jarring experience to go back and read the first issues or watch the first episodes.
Aoyama's style change is even more evident in his shorter manga, Magic Kaito. Considering that he only makes a chapter every few years, the series shows significant style changes in each volume (especially the 4th one, released 13 years after the 3rd).
Pikachu has undergone quite the style change as the Pokémon anime went on, as has Meowth. The quality of drawing is noticeably better, they're actually using CG in scenes now, and the art gets even better during important battles or when the animators just want to show off. The same applies to the games. The same thing can be said for the human characters as well, particularly concerning the eyes. Originally everyone had skinny eyes (except Brock), but as the series progressed the majority of the characters got more rounded eyes. Compare these two screencaps◊ of Ash doing his signature "Hat Turn", from Kanto and Unova.
Kotaku had an article featuring various comparisons from the Unova flashbacks of Kanto and Johto events.
Bleach has evolved significantly over its run. The first few volumes have a distinctive rough, square-jawed style carried over from Kubo's first series, Zombie Powder. By the time the Soul Society arc begins, the art style has become smoother and more detailed, with a noticeable increase in brush-style linework. That move towards more graceful and economical lines continues throughout the series, although slowly and subtly enough that most of the time it is not immediately apparent unless a character has been Out of Focus for a long time. For example, Isshin's appearances are rare enough that it's easy to see the difference in style from the beginning of the story, to just after Soul Society, to Deicide, to after the timeskip.
A compilation of initial character designs compared to later, evolved versions here. (Warning: Some Fake Karakura arc/post-timeskip spoilers!)
One Piece's distinct style has changed quite a bit over its decade-long run. In the beginning it used many thick lines, giving the art a round, bouncy, cartoonish look. The lines eventually became thinner and crosshatching and line shading is used extensively. The characters' features have become more loose to the point where Zoro, for example, can look buffoonish one frame and a hard-boiled Bad Ass the next. This carries over to many a character.
Chopper didn't start out looking as ridiculously cute as he does now... though he was pretty cute to begin with.
Looking back, the backgrounds and layouts were pretty bland compared to more recent ones. They were functional and well drawn, but weren't quite the feast for the eyes they are now.
This image◊ sums up the Art Evolution over the last 15 years. Luffy actually appeared more grown up at one point before once again becoming more child-like later on.
The Art Evolution in Naruto has been somewhat unusual. The drawing style becomes far more linear over time, making it a little less cartoony. This has an unusual effect on character faces. From straight on, they often end up being very flat, unemotional and generic. However, they are greatly improved in profile and at 3/4 angle. Also, in later chapters of the manga, the characters look boxy when standing still, as their bodies don't seem to taper at all at the hip (this has improved somewhat).
During the Tsunade retrieval arc, Kishi seems to have started making rather noticeable eyelash marks on the characters' eyes, giving many characters a temporary feminine look.
People who got into the series from the anime (which has art based on later chapters) may be surprised by how amazingly different some characters look in their first manga appearances. Shikamaru in particular was almost cro-magnon looking in contrast to later where he merely has a distinct looking nose and brow, and in that same chapter Choji's eyes frequently look like singular lines while later he's merely squinty.
It's even more noticeable with characters who only appear every once in a while. Compare the Iruka in an early chapter with this one. Gaaah.
Well, that had at least something to do with the lighting and the artist probably not being familiar with drawing him anymore, he looks far less creepy (though still different) in some later appearances.
Rurouni Kenshin also changed its art style greatly between the first volumes (somewhat amateurishly drawn) and the much more polished Kyoto/Ten Swords arc, then it went on to an even more stylized style for the final volumes.
And in the recent Kanzenban re-edition, the new covers and new character designs are once again completely different.
The art in the later volumes of X1999 is very different than in the earlier ones, not surprising since publication spanned over more than a decade. More generally speaking, with CLAMP you can follow their art evolution through their manga from RG Veda or CLAMP School Detectives to their latest works like XXX Holic and Tsubasa Reservoir Chronicle, since that they have alternated Mokona and Nekoi Tsubaki as main character artists in different comics. Overall, they have shifted towards using more elements of Nekoi's delicate style.
It's particularly glaring each time a character from one of their earlier works shows up in Tsubasa Reservoir Chronicle, looking somewhat different compared to how they did in their own series.
This is actually explained by the fact CLAMP likes to try to keep the art style in each of their series different from each other.
Kentaro Miura's artwork in Berserk has gone from being average to arguably being the best in the manga industry as it became more and more detailed and realistic. From volume 7, Miura has continued to refine his artwork, especially in the his characters' faces.
Dragon Ball started off with Toriyama's signature style with nearly uninterrupted lines, in other words almost no sharp angles. As the series progressed, the most notable change occurred mid-way through the Saiyan Saga of Dragon Ball Z and the characters were almost entirely made of sharp angles.
While somewhat subtle, Azumanga Daioh saw this happen. In the beginning of the series, Chiyo-chan's pigtails are shaggy, Mr. Kimura's mouth is closed, Mr. Tadakichi looks more realistic and Osaka has eyelashes and a more feminine cast to her face. It takes a while for the characters to develop what one would consider their signature appearances. Notably, the evolution continues, and near the end Chiyo-chan's eyes have a... distinctive look.
Azuma Kiyohiko eventually redid the series with the finalized art style for its rerelease in 2009. Have a comparison. (NSFW)
Making everything look like Yotsubato!
The art from the early (~1-10) volumes of Get Backers is remarkably different; the anime's character designs are based on the later volumes, so those who were introduced to the anime first often react negatively to the earlier, grittier character designs. The most dramatic differences are probably in Ginji, whose hair lengthened quite a lot, and Himiko, who started out looking very plain and almost boyish in her introduction story, but is incredibly pretty by the time we hit the Eternal Bond arc. Something in the way her eyes are drawn and her lips are shaded does it. That Rando Ayamine improved drastically over the course of 39 volumes is the general consensus.
Osamu Tezuka evolved from being a competent amateur to being the best artist in the field he helped invent. Compare this◊ from Diary of Ma-Chan (his first published work) to THIS◊ from Ode to Kirihito made about 25 years later. One is comedy and the other horror, but damn.
ARIA: Kozue Amano's artwork evolved considerably compared from her short story work to AQUA, and AQUA to ARIA. The anime also has this in every season, possibly with budget increase.
The frogs in Keroro Gunsou became less tubbier as the manga progressed. Sgt. Keroro and Pvt. Tamama look like they ate too many sweets in the past. Meanwhile in the anime the way the frogs have been drawn has changed slightly too.
While some of the characters have stayed relatively similar in design over its run, America originally started out with sleeker, center-parted hair. In time, he would wind up sprouting an Idiot Hair and his overall hair style became a bit mussed with a side part. Italy's own curl also started relatively small, but quickly grew into the defining characteristic (the difference was even poked fun at in an illustration).
France originally had shorter, somewhat greasy-looking hair, and the early art style definitely didn't do him any favors. While his hair grew out with the style change, he also lost the white spats over his boots.
Estonia and Lithuania had slightly different hair styles in their debuts, with Estonia having less of a bowlcut and Lithuania having less fringe. In comparison, Latvia started out as slightly smaller than the other two, but wound up shrinking and becoming more child-like.
South Italy was originally somewhat taller than his younger brother as both a child and adult, but wound up shrinking to become the same height.
This picture (click to zoom), charts out the evolution of the artwork.
Season 5 of the anime had a HUGE art upgrade from the seasons before it - the colours are more sophisticated and stick closer to Himaruya's colour charts, the hands actually look like hands (not hand-shaped-blobs) and generally look better - though  on whether the 'added girliness' to characters like England is an improvement or not.
The art style used in Beyblade changed after season 1 to a softer style that made (some of) the characters look younger... and then changed again in season 3 to give the characters edgier and more grown-up looks. And the animation quality was much better.
The Soul Eater manga's art has grown significantly more refined since the first volume. Notable differences are the generally softer and rounder lines and shading and specifically Maka's design, which was originally much more child-like. The anime adaptation took this eventual change and used it from the start for a more consistent look. There's even a difference from the pilot chapters to the first one. Compare this to this. Also, the number of Panty Shots and nudity scenes have toned down considerably, though the occasional one is still tossed in for the sake of Fanservice.
Vinland Saga, when it changed from being a weekly Shonen to a monthly Seinen, the art became much more detailed and a few of the character designs were tweaked. Most notably with Bjorn, a character that went from being a slightly pudgy Big Guy who could easily be described as fat, to a burly wall of berserker muscle.
In the first season of Digimon, Leomon's special attack was animated as an orange lion's head flying toward the target. In season 3, it was animated as a stream of fire beginning with a lion's face.
The manner in which the characters were drawn in Digimon Savers was also different to the manner they were drawn in earlier seasons.
D.Gray-Man started off as a sort of generic-looking manga but has since developed a more distinctive look. The art has become much more detailed and dynamic, and the characters (namely the Noah) have gotten much... prettier. No, seriously, see what happened to Tyki and Road for yourself: this to this and this. Some fans lamented the loss of "shota Allen," though.
There's a compilation of the art evolution for Allen over the 19+ volumes here◊.
The art also changed noticeably between chapters 186 and 187, when the author went on hiatus due to illness. When it came back, it was much more detailed and some of the characters had changed again; Allen and Tyki (who's also had an Expository Hairstyle Change) for comparison.
Although Tyki has returned to his original look, likely due to fans' requests.
My God has Katekyo Hitman Reborn!'s art changed... vastly. All the characters, as the series goes on, get handsomer and handsomer, and "ugly," "no-good Tsuna" has somewhat evolved into a cute little boy catering very much to the fans that like pairing him with the other men. Pictures of Tsuna's evolution: From this to this and then to that. The Big Bad for the 2nd arc, Xanxus, gets a big makeover 10 years into the future... from this to this and this.
For an even better view of Tsuna's evolution, compare his Dying Will Mode from the first chapter to the one from Chapter 407. Worth noting that the author had stopped drawing such form for over 300 chapters and only recently had used it again.
Gokudera's gone through VAST art evolution in the manga and in the anime. Just compare this to this.
Since pretty much all of the links are now dead, here is a quick runthrough of a handful of characters from their initial appearances to how they look now. Feel free to gawk.
While not really the case in the anime (being based on later novel designs), the character designs of the Baccano!Light Novels have noticeably changed over the course of the series — especially in the case of Isaac and Miria, who started out rather◊ sleazy◊ and end up looking like poster-children of hyperactivity that they are within a couple of books.
The art in the Yu-Gi-Oh! manga changed pretty drastically after the first couple volumes.
If you look at the cover art on the bunkoban releases of the manga, Takahashi's art has gotten much more realistic-looking. Here ya go.
Between Volume 1 and Volume 5... Jonouchi's hairstyle changes from a Leonardo DiCaprio-esque style to the signature Jonouchi style, and the general appearance of the characters changes. The Other Yugi appears increasingly less evil. Originally, Yugi was often drawn without a nose.
Honda looked pretty thuggish in the first volume. Kaiba was kind of short (compared to him towering everybody later on) while Mokuba was a rather chubby and really mean-looking.
Ken Ishikawa: Just look at the first series of Getter Robo (made in the 70s) compared to the second series, Getter Robo Go (made in the 90s). Back then his style was almost identical to his mentor, Go Nagai, and had lots of weird, warped proportions and expressions. As time passed, he retained the intensity of expression and sense of movement and refined it with stronger anatomy and greater detailing to create one of manga's most distinct art styles.
Eyeshield 21's art evolution is more noticeable with certain characters. Hiruma in chapter 1 kind of looks like Eddie Munster, but when that scene was redrawn for a flashback he's lost some of that feral look. Monta too changed over the course of the series. The evolution is especially noticeable with the Book End, where a two page panel from the first chapter was redrawn with all the new team members and showing how both the characters and the art progressed.
Similarly, characters that were once incredibly distinctive in their Gonkiness were toned down considerably as time went on; Kurita became less cartoony, and Niinobu Kasamatsu became about 1/3 his original width.
The Hellsing manga's style changes considerably over time. In the first volume, most characters have a generic manga style with huge eyes and tiny chins and not many unique facial features. The characters also have a tendency to be inconstant and facial features warp from panel to panel. This changes once Hirano gets comfortable with his style however. This is a character in 1 and here's what she looks like in 7.
The Higurashi no Naku Koro ni anime underwent a significant style change during the OVA Higurashi no Naku Koro ni Rei, greatly improving on almost all the anatomical errors present in the first two seasons of the anime.
Before that, Kai got a massive face lift when compared to the first season, which was often rather flat and rough. While this did occasionally work for the Sanity Slippage and Laughing Mad scenes, most of the time it was simply Off Model to the point where there was no model to begin with.
It's especially noticeable when they had a flashback to one of the more memorable question arc scenes, the scene where Mion and Rena visit Keiichi while he's "sick". They look cutermore welcoming◊.
Art Evolution appears in the manga when the same artist draws two arcs (question and answer). Like the Onikakushi artist. Her art style changed, and she got Mion's bangs right.
YuYu Hakusho saw its share of this, the manga and the anime. Hiei was the most significant example.
Kinnikuman's artwork began in a very simplistic, cartoony style befitting the comedy focus of early chapters. When the author genre shifted the primary focus from silly jokes to wrestling, the art becomes drastically more defined; characters have an actual shape rather than being mostly blobs.
Not only the art in Angel Densetsu evolved considerably during the series, the author usually talks about how he's refining his drawing skills in the tankobon's notes.
To take things into perspective, compare this◊ and this◊.
Bakuman。's art has changed, but the mood itself is the most noticeable. Everything at first, from the expressions to the staging, are all rather low-key. As the hero gets a more Hot-Blooded attitude towards his work and life in general, the art gets much looser and more energetic (but retains the careful attention to detail).
Author Kouji Kumeta, best known for Sayonara, Zetsubou-Sensei, has a habit of starting of starting off series in one style, and ending them completely differently. His first relatively successful manga, Go!! Southern Ice Hockey Club, started out in a fairly generic late-'80s style and worked its way into a much more angular, completely different look. Following on that, Katte ni Kaizou started out with this angular, shaded look, and ended up as, well... Sayonara, Zetsubou-Sensei. His latest series has had a modest amount of change in the way characters are drawn, but not as extreme as his previous two series. This is lampshaded substantially in episode 2 of Goku, where everyone is drawn in the author's art style circa 1991.
Somewhat apparent in Venus Versus Virus. The characters round out more evenly after volume 5. Very apparent when you look at the oneshot from the first volume.
In Rosario + Vampire, if you look at the first chapter and the most recent one without reading any of the ones in between, you'd think they were drawn by entirely different artists. The characters looked much rounder and more cartoonish (and just plain not as good) in their humble beginnings, but given the series began as a romantic-comedy, this evolution is plenty justified when Cerebus Syndrome set in, turning the events of the story into a full-blown shounen direction.
This is an especially striking example due to the astonishing speed with which it happened. Rosario + Vampire is 9 years old, but by its 2nd serialization less than 4 years later it was barely recognizable.
Compare this with this. No, seriously; that's the same artist, same series, only five years apart. Then, three years later...
Yami No Matsuei, where the manga artwork and the characters — especially the male characters — start out damn pretty, then through the first 11 volumes gradually became drop-dead gorgeous and sexy. Unfortunately, the very last manga installments suffered from a massive drop in art quality, with the character designs becoming distinctly crude and blocky compared to the earlier artwork, possibly because of health issues with the author.
Observable in Video Girl Ai (and probably continuing through all of Masakazu Katsura's works as well).
I"s, a later work, undergoes intentional art evolution. Early chapters, based when the characters are in early high school, use a shonen "typical" manga style. With successive chapters, as the characters grow older and more mature, so does the art style. Towards the end of the series there is a flashback to early on, complete with original art style. It's quite a shock compared to what you have gotten used to seeing over time. (See the example in the title's article.)
Since season 1 of the anime version of Slayers came out in 1995 and the most recent (Evolution-R) in 2009, obviously this would happen, but even watching the season 3 (TRY), which came out in 1997, then going back and watching some of the early episodes can be rather jarring. Lina's design in particular has changed quite bit? Most noticeably, her hair started out as reddish-brown and over time changed to bright red. Interestingly, though in all continuities there is a running gag about Lina having a flat chest, it started out rather large in the anime but actually see to have become smaller over time (though it still isn't as small as it's made out to be).
Rave Master was Hiro Mashima's first big manga, and he never worked as an assistant for anyone, so the style at the beginning is a little crude, and improves drastically over the 35 volumes the series runs for. In addition to everyone's faces looking less pudgy, Elie stops looking like every other girl who appears, Haru's hair spikes up more, Plue's ability to emote improves, and everyone under the age of 30 actually starts looking their age, rather than anywhere from 3 to 7 years younger.
The shift in Fairy Tail isn't significant, but if you compare the first few chapters with the current ones, Mashima definitely has a better idea of how to draw Happy now.
Monster by Naoki Urasawa. The art in the beginning is a bit flat and somewhat cartoony with the characters. Later volumes the characters look like they have more depth and more realistic looking. See Inspector Lunge in Volume 1  and then in Volume 18 
The character Johan Liebert goes from looking like this to this in Volume 13 (a flashback of that same scene).
K-On! started with the characters' pupils taking up all of their eye, and the lines gradually became thinner, cleaner, more solid, and were on-model.
The anime seasons also had their own evolution. Season 2's characters' models are more consistent, their heads are ever so littler in proportion to their bodies, and particular animation sequences make those in the first season look like chicken scratch!
In the beginning of the manga, Mugi didn't have distinctively large eyebrows, though they were slightly thicker than the others to give them a "blonde" appearance.
The Strawberry Marshmallow manga has this in spades; in the first volume the characters change almost unrecognizably between the earlier and last chapters. This may have to do with the fact that two years had passed by between making them.
Yumi Tamura's Basara underwent quite the change from its first volume to latter ones. The art began rather rough and angular but became more round and defined.
Hana Yori Dango began with lackluster art. Tsukasa's hair looked odd, but as the volumes progressed, the art got better, and it looked less like something on his head and more like hair. The eyes and faces also underwent a few changes.
Pokémon Special changed artists at one point. The first artist had a chibi-ish style with simple backgrounds, but she eventually got sick around the middle of the 3rd arc. The second artist initially attempted to mimic her style. It started out rather rough, but as time went on he smoothed things out with his own style, a more typical shonen style with more detailed backgrounds.
Future GPX Cyber Formula originally started out with average-looking art design, particularly the car designs, which looked more like toy race cars than real ones. But by the time the OVAs rolled around, the character and mechanic designs improved significantly, and the cars now looked like real race cars. Strangely, it also mixes with Art Shift in the last 2 OVAs, where some of the flashbacks has kept the older art, so some viewers might be confused.
The series' last two OVAs, SAGA and SIN, has a change of the art style. The clearest example of the trope is Asuka Sugo, the protagonist's fiancee. She looks clearly like a full-grown woman, but since several flashbacks and photos of her younger days keep the old art, Asuka ends up having 2 different looks in the same show. For an example, here's her look in the TV series◊ and here's her look in SIN.◊ This is because the last 2 OVA's had MyHime (and My-Otome's) character designer.
The Big O had a significant art upgrade between its two seasons. This may be in part because season two was drawn on computers; legend has it that season one was the last major traditionally drawn anime.
The artwork of famous character designer Akio Sugino has changed drastically over the years. Just compare this shot◊ from the 1973 version of Ace wo Nerae to this one◊ from 1988's sequel.
To be fair in the earlier adaptations of Aim for the Ace!, he was basically copying Sumika Yamamoto's artstyle (y'know the creator of the original manga), in the OVA's the series ArtShifted over to Sugino's own artstyle
The first volume had a comparison with a protagonist of a previous series by the mangaka. The latter relied on Artistic Age, while the former had realistic looks for their ages. The protagonists of said manga were middle schoolers, not that much older than the Wandering Son gang near the beginning of the series, but are lanky and look years older than they are. Compare this◊ to this◊ and this◊.
Colors have changed in Wandering Son. Takatsuki originally had brown hair before changing to black around volume 3 and Shi originally had blond. Everyone has Curtains Match the Window's, so the brunettes have brown eyes and the raven haired characters have black eyes; Takatsuki changed eye colors in volume 5 and Chiba in volume 4 (apparently Anna too). Nitori's hair color has darkened over time too, from light brown to a darker shade.
You're Under Arrest!'s most noticeable change was between season 2 to season 3, though there's a noticeable change (de-evolution) from the OVAs to season 1. The series has gone through several design changes during its run, each to fit the style of the time period. Most noticeable being the eyes which have gotten more big, colorful and bright. Aoi gets a noticeable change◊ in appearance over the pace of the series.
Very apparent◊ in the manga, where originally the characters looked nothing like their iconic designs.
The main evolution Good Witch Of The West took was between the first and second volume when the artist abandoned the childlike figures and to draw them more realistically. The male characters took quicker form than the female characters. Along with this the artist drew less and less flowery backgrounds as the series got more serious.
Elfen Lied's first volume◊ was extremely Off Model and very 90s-style. The manga however quickly became more high quality, to the point where by the last volume◊ it could pass as being drawn by a different mangaka; it also looks like it went from Shōjo (Demographic) style to Shōnen style. The Diclonii's horns were also a lot longer originally, though later they started to look like cat ears.
Slam Dunk went through extreme jumps in quality for a manga that only lasted 6 years (relatively short by Shonen Jump standards). The style of the early chapters was generic and dated; thick lines, too much shading, and pompadours everywhere made it look like any late 80s and early 90s shonen manga. By the end of the manga, Inoue has his trademark realistic style, making Slam Dunk distinct from other mangas by the mid-90s.
Noblesse has had 2 major art shifts. According to fan polls, it gets worse each time. If it didn't have the same artist's name on it, you'd think they were drawn by someone else.
The anime of Sailor Moon is noticably different between the first and 5th seasons. Just try watching the first season after finishing the 5th. It isn't simply better animation, the characters look noticably different (to be fair they're supposed to have aged 3 years). Usagi herself has become rather sexy by the end.
Early chapters of Kodomo no Jikan featured highly simplistic, flat character designs and little shading. As the manga has accumulated more installments, the artwork has become noticeably cleaner and more detailed.
In Black Lagoon, Revy used to look a bit more Asian in the first arc, and her hair was a good bit shorter.
Despite having fought in the Soviet Invasion of Afghanistan, Balalaika looked around 20 years old in a series that takes place in the mid 1990s. By the second arc though, ten years seem to have been added to her age.
Hisashi Eguchi's art style has changed dramatically since The Eighties. Nowadays he uses a more signature, realistic style compared to the more generic style he used then. Compare the way he originally◊ drew Hibari, from Stop Hibari-kun, to the way he redrew◊ her on the covers of the recent reprint.
Unusual anime example with Little Busters: the animation of the first season was pretty regular, with occasional Animation Bumps for big episodes. (Though not the first episode, strangely.) Naturally, the people who were angry Little Busters was being animated by JC Staff and not the illustrious Kyoto Animation were not happy. And then the second season, half the length of the first, came around, and just the first three episodes alone showed much, much better animation than anything from the first season. It's as though they got the same animation budget for both seasons and used the excess damn well the second time around.
The GeGeGe no Kitaro characters have changed more than a little over the over 50 years it's been running. Compare Neko Musume◊ over the years. In contrast, Kitaro◊ hasn't changed much in design but you can see the art style differences.
Isayama Hajime's artwork for Attack on Titan has improved vastly over time, becoming more consistent and just plain better. Even fans will admit the early artwork was dreadful, with rough pencils and characters that looked awkward or hard to tell apart. Comparing the early chapters, with its incredibly awkward and inconsistent work, to the more detailed and flowing recent work, shows just how much his artistic abilities have improved with practice. Compare this early group shot to this later one.