As the title says, I'm looking for stories with bullies who change their ways. I'd prefer characters who manage to change without themselves being bullied (like Koe no Katachi), but rather in some other way grow and realize the error of their ways. Additionally, I'm using "bully" in a fairly broad sense to include not only people who deliberately beat others for their own benefit but also people who help others in the bullying in one way or another.

Closest I can think of is My Hero Academia's Bakugo, but even after he reforms and becomes pretty close to a "friend" he's still clearly very contemptuous and eager to crush those around him; he's got quite a ways to go.

It's been a while since I read it so it might not be exactly what you're looking for but I think the main theme is in the range of what you're talking about, you might try Attaque.

Good luck ?

Fukanzen na Hito no Tame ni
Sakamoto Desu ga?

~4400 manga completed
~1700 ongoing...

Seconding Sakamoto Desu ga? though be warned that it is comedy.

Adding:
Mahou Shoujo Site
Nanohana Boys
Stalker, kind of.