Jason, the protagonist of Long Ago And Far Away was once the heroic "Child Knight" who saved the land of Elvenwood from the evil Witch-Queen Nexis. In the years that followed, he grew up into a 30-year-old Jerkass.
Natalie "Envy" Adams from Scott Pilgrim Volume 3 took a several year course in jerk-assery and passed with an A+++. In college, "Nat" used to be a nerd who had a lot of anime stuff in her dorm and kept to herself. She was Julie's roommate and she ended up meeting Scott and dating him for a nice while. Her, Scott, Stephen Stills, and two insignificant others (one being Young Neil's older sister Stephanie) started a band called "Kid Chameleon" that ended up becoming pretty popular, eventually cornered by record labels. Over time, she began becoming more confident and selling her old possession for cooler things while slowly becoming more of a 'bad girl,' even using the alias Envy. However, she also ended up changing the band without the consent of Scott or really anyone with the rest of the band against the changes. In fact, it's implied that Envy may have begun cheating on Scott and certainly saw less and less value in ther relationship. Eventually, an incident at the bar led to a drunk Scott and Envy creating a large argument that Scott didn't recall anything, but Envy announced that it was over, both the band and their relation, causing heartbreak for Scott and a good deal of insecurity. Byt the time we see her in the main line, she has become pretty successful though Scott has compliations talking with her while Wallace is pissed off for what she did to Scott.Now in a band with Todd Ingram, her now boyfriend, childhood friend and Ramona's third evil ex-boyfriend, and a drummer named Lynnette Guycott. In bitter irony, Envy discovered Todd himself wasn't as loyal as she thought and with Todd's defeat and Lynette's disappearance, she was left with little else but the clothes on her back (including an old hoodie from Scott). The two didn't make closure until the final book where Envy seemed to have found success going solo, albeit being with Gideon. With his end though, not sure what happens to her, but at least the closure helped repair the damage between the two.
Age of Bronze: In A Thousand Ships, Paris starts out as a slightly arrogant but relatively decent and enthusiastic youth who is a bit overwhelmed by his new life. By Betrayal, Part One, after three years as a Prince of Troy and being indulged in his behavior by Priam, he has become very arrogant, rude, pushy, and refuses to accept he could be wrong.
In Sonic the Hedgehog (Archie Comics), King Acorn in the pre-Continuity Reboot universe spiraled into this direction after the restoration of his body, disbanding the more battle-hardened Freedom Fighters to install his Secret Service and trying to force Sally into marrying Antoine, but went full tilt after his body was damaged by Antoine's Evil Counterpart Patch.
Hot Rod after becoming Rodimus in IDW's run of The Transformers. In The Transformers: Autocracy, his main concern was protecting the poor Cybertronians who lived in the slums of Nyon. By The Transformers: More than Meets the Eye, he's become an egotistical jerk who in love with the sound of his own voice and loves being in charge even though he's a lazy slacker who pretends to be asleep whenever someone approaches him with something that bores him.
Huey, Dewey and Louie. One story had them complaining to each other about Donald Duck's many flaws only to suddenly realize that no matter if Donald is a big success or not, he still takes care of them, cooks for them, makes sure they're clothed and happy, and on the whole uses a lot more money on them than he does on himself. Cue guilt and a resolve to start appreciating their uncle a little more in the future. Later stories have them be openly ashamed on him and even come off as selfishJerkassesnote In "Where There's Smoke", one of the three said he wasn't annoyed by Donald dropping foam on them (because Donald thinks that where there's smoke, there's fire), but because he would get the badge instead of them, and the comic portrays them in the right for their It's All About Me attitude, and they even get rewarded for it instead (they get their badge at the end), and in "Paperino e i Gamberi in Salmì", Scrooge's brother Gedeon gives Huey, Dewey and Louie a stand-in of the "Pulitzer" prize, but gives it to Donald to keep it for them until they grow up. Afterwards the three are shown running Donald out of town and saying "You're not our uncle... you're a medal snatcher!" just for getting the medal, which he didn't steal it from them., and it doesn't help that they stilltake the moral high ground against Donald and win.
The Life and Times of Scrooge McDuck shows Scrooge McDuck's decades long quest to become rich turned a nice, hopeful kid into a cynical misanthrope. As commented on by his sister Hortense in The Empire Builder From Calisota:
Hortense: Getting richer and richer, and meaner and ornerier! That's all you do.
Asterix: After spending the entire series as a joke character, Brutus suddenly becomes the Big Bad of Asterix and Son. He goes further than any villain previous and burns the village down. Not to mention, it's heavily implied he was trying to murder his baby brother in order to remain Caesar's only heir.
Battle: The Scholar becomes more obnoxious after Grogan's death, leading Charley to realise that Grogan might have been right about him thanks to becoming pally with Snell, who helps him with an application for officer training. When he returns to the front as a 2nd Lieutenant, he is outright snobbish towards Charley.
The title character of Funky Winkerbean was something of a pessimist as an adult, but still mostly a nice guy. After Time Skip #2, he became a grumpy, avaricious Jerkass.