Peanuts was a little more detailed in the earlier years before evolving into Schulz' signature minimalism. Characters and objects were never shown from anything other than front or side views, and the face/head designs changed from ovals to circles. This further evolved a looser, scratchier style due to Schulz losing fine motor control as he aged.
Garfield famously went from a thick-bodied, beady-eyed creature to the more symmetrical, goggle-eyed form seen here. Ironically, even though he's a cat, the original design of him looked more catlike than his refined design. Also evolved was the style - the strip originally looked slightly more realistic in its designs of people and animals. By 1983, the style became more cartoony.
And Jim Davis' other comic, U.S. Acres, went from a cutesy, round style similar to early-80s Garfield to a loose, cartoony, frenetic style.
Compared to Peanuts and Garfield, the change of Calvin and Hobbes' actual character designs was relatively small. However, as Bill Watterson's art improved by leaps and bounds and the strip was granted a larger Sunday format, lineart became crisper and less sketchy, dinosaurs became more realistic and anatomically correct, and the backgrounds became more detailed.
The alien desert scenery in the Spaceman Spiff strips began as looking quite abstract and cartoonish; they soon gained a better sense of scale and became more realistic, rugged, and natural-looking (drawing inspiration from the landscapes of the American southwest), and ended up a great deal more visually impressive as a result.
For Better or for Worse changed artwork styles wildly over its run. From a rough looking beginning (no background, slightly odd-looking but serviceable character design) the characters and background became slowly much more detailed and realistic to the point where the attention to detail and shading seemed extremely obsessive and a little intrusive. But interestingly, now that the comic has been rebooted, the artist has had to relearn her old art style, so that it doesn't force a contrast between the new strips and the reruns.
In the ten years who Mafalda lasted, Quino's drawing went from crude and slightly Off Model to refined and fine-line drawing, with a more consistent character design.
FoxTrot was a lot looser and more detailed in its first couple years, with more realistic backgrounds. Throughout most of the 1990s, it gradually became more flat and cartoony (most notably, there was more of a "gemoetric" feel — eyes became straight circles, the strands of hair over Jason's glasses became evenly spaced, etc.). In the last few years before the strip switched to Sunday-only, Bill Amend also began putting less detail in the background.
Luann. Lampshaded in the 25th anniversary Sunday strip, which had Luann, Bernice, and Delta commenting on the "twerpy little freshmen" who look exactly like them when the strip started.
Lampshaded in Ginger Meggs (although considering it's been going since 1921 and has had like half a dozen artists in that time, a fair amount of mutation would be inevitable). On a trip to Paris, Ginger has a caricature done and says it looks nothing like him; the caricature is definitely the original, when-your-grandparents-were-young artwork.
Bill Holbrook's strips have evolved over time, with the oldest, On The Fastrack, showing the most change. Even his webcomic, Kevin & Kell, shows gradual shifts in how he draws the characters, syncing up with how his art for his syndicated strips changed over time. A flashback gag in 2010 for On The Fastrack showed the current cast as they looked in the 1980s.
Funky Winkerbean, in its earlier years in the early 1970s, used a much more cartoonish and loose art style matching the "gag-a-day" nature of the strip. As Cerebus Syndrome took hold, the art also became far more realistic — though this is also when the cast developed almost permanently depressed facial expressions. A flashback storyline in 2010 in which the middle-aged Funky meets his teenage self places the drastic art style change in much starker light, as even the youthful Funky was drawn in the modern style instead.
Alley Oop did a Lampshading story arc a few years ago where Alley's no-good cousin Early Oop shows up in Moo and makes trouble by impersonating Alley, despite Alley's insistence that they look nothing alike. Naturally, Early is designed to look like Alley's original appearance, all those decades ago.
Eek and Meek by Howie Schneider was consistently well-written and funny throughout its run, but artwise, the characters eventually became so abstract that a new reader probably couldn't tell they were supposed to be mice without being told.
Eventually in the early 1980s Howie Schneider decided to just turn them into humans. It was never brought up in the strip until the next-to-last comic, where Eek said to Meek that he wasn't a bad mouse either in the beginning.
Bloom County is also a very extreme example. In the first year, the art was very blobby and scratchy, and then it started to ape Doonesbury for a while — something that even Berke Breathed himself admits to. Over time, it gradually became much finer and clearer, with Berke putting more detail into his inking and crosshatching at times. The fine, crosshatched style carried over to successor Outland, and by the time he made Opus, he even changed up his coloring style drastically.
Opus the Penguin had his own Art Evolution, looking very much like a penguin when he first appeared as a minor character, only to grow a much larger beak/nose for no particular reason as he became the central character in the strip.
Popular Norwegian comic Pondus made a huge art shift from flabby caricature drawings to a more realistic style early in its run. Compare this early strip◊ to this recent one◊... and yes, the black-haired man is the same character.
The Filipino comic Pugad Baboy show shifts through this during The Nineties. What started out as a sketchy, three-panelled strip turned into a more rounded and detailed version satire.
Little Orphan Annie used to have huge red hair which was a white girl's afro, which later shrank to a realistic size. And the rest of the art grew more consistent and less sketch-like, too.
Over the Hedge became somewhat less scratchy-looking over time, with the characters' designs also morphing — most notably, RJ's head changed from rounded to a more realistic raccoon appearance, and Verne's nose became larger.
Beetle Bailey gives an example that has been around since 1950, so that the art has had an opportunity to shift around for several decades even after it found its own distinctive shape. Most of the evolution — towards rounder, more stylised characters — took place within the less than ten first years, and during the sixties, the art reached almost exactly the form it would have from then onwards. Still, there are always small differences at different times. Insofar the art has changed since the eighties, it has mainly become sloppier in the 2000's. Somewhere along the way among the older comics (not necessarily strips at all, but slightly longer "stories") there is also found a version of the art style so odd that one wonders who actually drew those.
Baby Blues had more of a sketch-y look to its art in the early days before gradually becoming more polished and refined.
Dilbert's early strips are remarkably amateur. Scott Adams didn't have any real cartooning experience at the time.
Marvin had a lot rounder, almost Garfield-derivative look in its earlier years. Nowadays, it's noticeably more jagged.
The Family Circus originally had a stiffer, more jagged look, as seen here◊. Also, the dad was originally an overweight buffoon who smoked, drank, and wore a hat. Around the 1970s, the art smoothed out to resemble the style used for the rest of Bil Keane's run on the strip, and the dad took on his present personality and appearance.