For a while in The Sixties, classic comic strip Dick Tracy tried to capitalize on the ongoing Space Race by sending its characters TO THE MOON! One character even got married to a "Moon Maid". After the moon landing, however, the moon, and most of the sci-fi elements, were dropped from the strip, and Dick Tracy went back to old-fashioned crimefighting...
... only for the strip to almost immediately stumble into a second Dork Age with its faux-blaxploitation/"street-wise" '70s style, including giving Tracy a god-awful mustache and longer hair.
The strip would eventually hit a third Dork Age from 2006 to 2011, which is widely considered to be its absolute worst period. A combination of artist Dick Locher's advancing years and the failure to replace writer Mike Kilian after his death led to five years that were marked by atrociously bad artwork, and stories that tended to drag on for three or four months without much of anything happening, before everything was suddenly and unsatisfactory wrapped up in just a few days. Fortunately the strip was able to recover yet again when Locher left and was replaced by Joe Staton and Mike Curtis.
For Better or for Worse is widely considered to have ended on one of these, then restarted on another. It became a soapbox for the creator's very dated (and somewhat warped) views and beliefs.
Garfield seemed to be going through one for a long while, especially after having Jumped the Shark once Jon and Liz became a couple, destroying any future "Jon has a horrible love life, or has insane wacky dates" jokes. As well as most side characters (Nermal for instance) seemed to have been put aside, appearing very rarely, to put more focus on Garfield, Jon and Liz.
With B.C, near the end of Johnny Hart's life, he became very heavily Christian implementing his beliefs heavily into the comics, sometimes becoming very controversial. (One infamous strip had a menorah turn into a cross) After Hart's death, his kids took over and turned it back to it's original format.
Funky Winkerbean was simple enough, a wacky comic that took place in a high-school. Years later and after a few in-universe time skips, it became one of the most dark, depressing comics ever syndicated. Constant mention of cancer, death, characters trapped in a miserable world, and even a story arc of one of the major characters getting Cancer and slowly dying from it for the reading public to follow. Even it's sister comic Crank Shaft (Which is in the same continuity) can't escape the constant gloom that all the characters suffer from. This isn't an dramatic exaggeration either, read Funky Winkerbean's trope page.