James Howard got his start working on The Astounding Wolf-Man and his growth as an artist is very evident. In the early issues his art was cartoony and character anatomy was a little odd. Yet he improved so that by the last issue he was using much more shadow and his designs were significantly more intricate and detailed.
Naturally, every Golden Age character has gone through this. For example, Superman's emblem wasn't quite the shape we know today when he first came out. Also, Batman's costume was modified a lot in the first few stories and, appropriately enough, it wasn't until Robin debuted that Bob Kane had come up with all the main elements of the design.
Tintin improved enormously as Hergé developed his signature ligne claire style.
The Wolfrider elves in ElfQuest were originally quite short and stocky, since their design was influenced by the elves in Ralph Bakshi's Wizards. As the series went on the elves appeared to gain about 6 inches in average height. When the original series was reprinted by Marvel comics additional pages had to be added to fit Marvel's page count, and the difference in style between the old and new artwork is very noticeable. (The new pages were retained in subsequent reprints and the online edition).
Gold Digger has improved a lot over time, the linework improving very noticeably over the first 25 issues or so and continuing to get better in smaller ways, and the jump to color which started good and became incredible.
Likewise fellow Anarctic series Ninja High School did so as well. Starting off from a look reminiscent of Golden Age comics to a more cartoony look giving the characters wider eyes and less thick outlines.
Craig Thompson. You can tell Blankets and Habibi are done by the same guy, but the difference in skill and execution is amazing.
Frank Miller appears to be a complete inversion of the trope, with his quality taking a nosedive between Sin City and Holy Terror. In The Dark Knight Returns, Miller made the effort to include detail and backgrounds. But after Sin City, Miller made all of his art resemble the gritty minimalism of that work at the expense of background, detail, realism, and eventually any sense as sequential art, while experimenting with... questionable new methods. Of particular notoriety was his use of cheap Photoshop effects as substitutes for backgrounds in The Dark Knight Strikes Again.
Cerebus the Aardvark faced a drastic improvement in its artwork over the first 50 or so issues as Dave Sim moved from aping Frazetta-style fantasy art and started developing his own style. When Gerhard started doing the comic's background art, freeing Sim to concentrate on the characters, the art improved again.
Druid City: The series starts with a very simple, poorly penned style, done entirely with a computer mouse. The artist for the series originally wanted to place as many limits on himself as possible. The methods changed starting in Volume 2, where everything is drawn with a stylus and the artist has more experience with the personal style used in the series. Now that Volume 3 has started, the artist has also announced that Volume 1 will be redone with a stylus and more detail, so as to not scare away new readers.
Buddy Longway. Even if Derib's way of drawing landscapes has always been extremely detailed, his character design went from somewhat cartoonish to highly realistic and detailed during the years.
The entire point of the "Mick McMahon Collection", a collection of Judge Dredd strips bagged with Meg 301, see to have been to illustrate just how much McMahon's art improved. "The Howler" is crude and blocky, and it's often difficult to tell what's supposed to be going on. "Voices Off", on the other hand, looks like it was done by a completely different artist; it's very detailed, infinitely more realistic, and also nicely fluid.
Nodwick gradually got more stylised during its time in Dragon magazine. Most notably, in the early strips, Nodwick's nose and Piffany's glasses are both realistically sized. (One strip suggests Nodwick's nose has gained mass to balance out the stuff he's expected to carry on his back, and in an interview Aaron Willia says Piffany's eyes keep getting wider with shock at what her teammates get up to.)
Parodied/Lampshaded in a Dork Tower strip celebrating Dragon's 30th anniversary. The strip purports to show how the three strips then running have evolved over the years. A genuine early Nodwick, crude stick figures that are supposed to be "early What's New?", and an "early Carson the Muskrat" ... who is actually Yamara.
This becomes a minor plot point in Teen Titans. In Raven's early issue appearances, she looked every bit like an average teenage girl. But George Perez gradually gave her sunken eyes, a gaunt face, and receded her hairline, giving her a darker and more sullen look. Cyborg notices this early in The Terror of Trigon saga when he looks through some old photos, and is the first to realize her demon father's influence is taking over. After Perez left the title for other works, his successor Eduardo Barretto drew Raven more like her earliest self (although after he left, other artists took her back to her gaunt appearance).
In Generation X, Skin has the mutant power of extra skin that he can control at will, sort of a pseudo-Rubber Man. However, it left him grey-skinned and ugly in the beginning. But by the end of the book's run, he had this roguish handsomeness going on. It's never addressed in the books, but it's fun to imagine that as he found new ways to make his powers useful, he began altering his appearance, or that his improved self-image made him more attractive on the outside.
Archie Comics has gone through some serious redesigns over the past 50+ years. The art was simplified sometime within the 60s or 70s and occasionally Archie tries out a new design - to which they always return to the "classic" one in the end.
Noticeable in The Beano when David Law was drawing Dennis the Menace (UK) the character actually grew over the years from a small child who looked about 8 to by the 70s the character was lanky almost teenage looking character. The character's design stayed constant when David Sutherland took over drawing him in the 70s and then finally the character was made smaller and younger looking in the 90s in time for the Animated Adaptation.
Steve Moncuse's art in Fish Police took a more "shiny" appearance when the comic changed from black-and-white to color upon moving to Comico. Interestingly, he kept the color-friendly style even when it moved to Apple Comics and reverted to black-and-white.
In the German comic Werner: Can be watched throughout Oder was? and over the course of the first eight books. The drawings got clearer and more detailed. Eiskalt! let a shade of gray enter. Some stories from Normal ja! on were inked, yet still remained grayscale. Ouhauerha! was the first book in color, and when its successor Wer bret hat Angst! was released, the art had evolved so much that people decided this wasn't the Werner they knew and loved anymore, also because hardly anything was actually drawn by Brösel hielf anymore rather than the staff of artists he had hired meanwhile.
Phil Foglio's work on his own Buck Godot: Zap Gun for Hire can be seen to become more streamlined, distinctive and iconic with literally every single installment of the series.
Astérix, changing drastically within the same volume at times. Notably, Asterix and Obelix both started off quite crude and lumpy-looking but got more rounded, cuter features (Obelix also traded pointed helmet horns for small nubby ones, lost his body hair, and stopped carrying an axe); Cacofonix was altered from an older man to look like he was in his late twenties or so, became much skinnier and more angular, and his hair went from a smooth bob to Barbarian Longhair with a sticky-up fringe and finally to an entertainingly anachronistic '70s rock star hairstyle; and Fulliautomatix completely changes in both face and body from a rather plain overweight late-40s man with blond hair, to a late-30s, muscular, proud-looking character with hairy arms and red hair (although it's inconsistent). He also stopped wearing a shirt and replaced it with a leather apron. Obelix's dog, Dogmatix, also goes from a squarer, more terrier-like look with drooping ears to a more anthropomorphised, Disney-like appearance with raised ears, within the same story.
Whilst Judge Dredd has had many artists over the years whose styles have evolved, the most noticeable example was Mick McMahon, who went from being forced to copy Carlos Ezquerra's style after the latter left Dredd, the emergence of his unique angular style would shape the art for years to come, particularly his now-iconic re-interpretation of Dredd's uniform.
Luther Arkwright: Especially notable between The Papist Affair and the later stories. Heart of Empire also used very different techniques than the earlier stories, besides being in full colour.
This can be semi-justified in Noob by the fact that it's set in a MMORPG and character designs imply that avatars gradually change their head to body ratio from "super-deformed" to almost normal-looking as they level up. Fantöm, however, is already at maximal level when the story starts and is first shown with a slightly muscular upper body. Later on, the upper muscles became much more impressive and his legs very thin. Ystos is noticeably subject to that too, but it may have been done one purpose as his player gets introduced in an unexpected real life scene and taking the opportunity to update his design was the best way to keep the reader from immediately realizing that it was him.
Barry Windsor-Smith's art on Marvel's Conan the Barbarian went through an astonishing art evolution in a very short time, but this ultimately had a negative effect; Windsor-Smith felt that his increasingly detailed drawing deserved a higher page rate than he was getting (it was becoming harder and harder for him to keep up with a monthly schedule, which meant his improved drawing was actually costing him money) and Marvel wouldn't pay. He left the title after just a couple of years.
When Knights of the Dinner Table debuted in SHADIS magazine, the artwork was, frankly, hideous. Even when it got it's own magazine it wasn't much better - the characters were usually just staring out into space, their mouths hanging wide open to disgorge their speech balloons. The positions of the characters were also extremely limited, due to it's nature as a Cut-and-Paste Comic. Now, the art has improved enormously. The basic character models are still the same, but the expressions look far more natural and varied, the environments and objects have more depth, and they have a far more flexible range of motion, from standing to driving to Table Flipping.