QUESTION: My main character used to be a bad guy, but now he’s not. He’s gone elsewhere and changed his name. How much of his past should I include? Do I need to write scenes from his past? Will readers believe he has changed?
If his past (backstory) is important, and it probably should be, you don’t have to include scenes of that past unless you think the reader wouldn’t understand him or his backstory is really complex. Usually in a case like this, his past life must impact his present one, and backstory scenes are interlaced with the present day.
Remember that every time a scene from the past is inserted, the reader stops dead to get his mind into the past then must stop dead again to get back in the present. This kind of back and forth is not a good thing in popular fiction like fantasy.
Current TV shows that interlace the past and present are FOREVER and PERSON OF INTEREST. This interlacing works better on TV than in novels because the viewer is used to sudden breaks in action courtesy of commercials.
Backstory can be inserted easily enough during present time scenes through dialogue, thoughts from the main character, and events.
He could be in a tavern to meet another character and hear a drunk nearby talking about his former identity's bad-ass behavior and think — “He'd piss his pants if he knew he was sitting a few bar stools away from me." Then you could have another character say, "But (insert former name here) was decent enough. He'd never fight around civilians and that time he rescued the child from the burning house instead of taking the money. You wouldn't see (insert new bad guy's name here) do that."
Sooner rather than later, you’ll also need to tell the reader why he chose to change. Again, it need not be a huge info dump.
As to whether readers will accept a bad guy as a good guy, part of this is determined by genre expectations from its readers. A truly despicable character would never be accepted as a hero in a romance, but elsewhere readers have a lot more forgiveness about this. In your reading of the genre you are writing, do you recall characters who switched moral sides and did it work and why?
Two superhero movies I can recall where the bad guy turned into the good guy are MEGAMIND and DESPICABLE ME. The change in their characters was the story.
And think also of Magneto in the X-Men series. As a bad guy, he is morally and emotionally complex, and he's helped his former friend Charles Xavier more than once to save the day for everyone's sake.
Usually, bad characters who change sides have already shown they are capable of good behavior with the bad behavior. That makes it more believable. A sociopath who changes to become a hero is totally unbelievable.
The trick is making your character's choices and changes believable. If you do, the reader will accept them.