Dark Elf Drizz'l of Eight Bit Theater. In his very first appearance, he was stupider than Garland. By his second appearance, he'd already started drifting into his current Only Sane Man and Deadpan Snarker characterization. As with most such things in 8 Bit Theater, author Brian Clevinger did it because it would be funnier - having one completely sane character was funnier than one more complete idiot in a cast full of complete idiots.
The protagonist of the webcomic The Lounge, Italy Ishida, seems to get a pretty large dose of this, cross pollinated with No Bisexuals.
Matt in Concession went from being a shy and polite virgin Christian boy who happened to be bisexual into a sex obsessed, diaper wearing, cross dressing, BDSM junkie who has sex with transvestites behind his boyfriends back and uses dark magic to fight priests who are trying to save the world.
Lord Sigfried von Damaske in Dominic Deegan. His original character was a Good Is Not Nice bully, but he began trying to improve his temper and people skills. Then, author Mookie got tired of people liking Sigfried, so he turned Sigfried into a racist and killed him. When people insisted on still liking Sigfried, he wrote a story arc where Sigfried had gone behind his friend Milov's back to sleep with Milov's girlfriend Jayden. After that, he wrote that all of Sigfried's earlier character development had been completely phony and done with the sole intention of getting into Jayden's pants.
In Red String, the author suddenly fell in love with a different love interest for the main character, Miharu, and had a problem in that she'd invested a lot of time in the original male lead, Kazuo, and how they were destined to be (as in the red string of the title). Near the end of the comic, Kazuo suddenly devolved into a creepy, awkward loser that "couldn't let go" - but as he rightfully points out, Makoto is a terrible person and in the very scene where he says this, Makoto has literally yanked Miharu away from him and yelled at her for continuing to associate with him. Naturally, as people still persisted in hating the Creator's Pet that was Makoto and still expected her to give Kazuo a fair shake, Gina portrayed Kazuo as forcing himself on Miharu as a last ditch effort to appeal to her. She wrote in an author's note on the original page that she did so to portray the scene as having "consequences" for him. He had never been portrayed as being this desperate and the act was clearly intended to contrast negatively to Makoto. The backlash from the original page (which showed Miharu actually wiping the kiss off) was so powerful that Gina eventually tried to redraw the page, but most of the fans still saw it as the Jump the Shark moment it was. Note that Makoto, despite constantly forcing himself on Miharu while she was dating Kazuo, is brushed off even by the woman he cheated on for Miharu as him just being a "flirty goofball." And in the final slapped together epilog that the author only wrote to get people to stop bugging her about what happened to Kazuo? Oh, she shows him as happy that Miharu has completely blown him off...when mere hours earlier, he was declaring that he still loved her.
A bonus story for the omnibuses that was written in the final year of the comic when full Creator's Pet status had set in for Makoto was noted as portraying Kazuo in a much more negative and awkward personality than he had previously been portrayed as in the start of the comic. Simulataneously, Makoto was shown as being a much nicer and kinder person than he actually was at that point in the comic. A lot of readers saw it as Gina trying to further retcon the characters to make the audience like Makoto more and Kazuo less.