What happens when a world champion switch weight classes?

So in boxing when a world champion wants to change his weight class, does he have to give up the belt to the previous weight class, or can he keep it in the new weight class?|||If a fighter holds a title belt in any weight division they have to defend that belt in whatever weight division it is that they won the belt in, should they move up and switch weight divisions they either give up that belt or it is possible that they can bargain and try and hold more than one belt in different weight classes, ie. Henry Armstrong at one time held world titles(this is when there was only 1 champion for every weight division then only 8 total weight divisions) in 3 different weight classes simultaneously.|||So in boxing when a world champion wants to change his weight class, does he have to give up the belt to the previous weight class, or can he keep.|||The champion must vacate the title...nuff said !