what would happen to a thief who learns their lesson and becomes unselfish and even generous after a lot of development? would their class change?

The fact Thieves are powerful Solo Classes and can have pretty bad tendencies doesn’t mean all of them are awful and can’t cooperate. Meenah and (Vriska) have great moments, a Player’s Class will never change because they’re at a better spot, the Class and Aspect reflects their TRUE NATURE and any deviation from that will likely not be healthy. A Thief being generous and sharing can happen, if they want to accomplish a goal and have their friends backing them up, they can share the spoils to reach a common goal.

Not to say, Class stuff is Guidelines and not Set in Stone.