what would you do if a lot by time friend turned into a bully?

Teach them a lesson.

I don't think it matters the length of the friendship if you can't talk to them about it then disengage it's not going to get any better if he will not admit anything.
Might have one conversation with him but if it ended up with him bullying me ,I'd be done.
Not saying it's not hard to disengage from a friend because you have a lot of nostalgia I get that but he's not treating you like a friend should.

Bully them back you’re as you are adults unless it is harmful to you

I’d try to find out why they were acting that way if possible. They might be unhappy and there could be something you could help with.
I was a bully in school.

This happened to me quite recently. I decided to tell him calmly that I have my own boundaries and cut him off for good. Never again.

tell them off

you can mirror the behavior the moment they do it.
Then immediately after tell them I just mirrored their behavior and wait to see their reaction. If they can open up and say something or if they want to fight me more.