The operation that renumbers the belt is called “conform” and is a separate op from branches. It takes a belt number list, and those are moved to the front of the belt, in order, and the rest of the belt is invalidated. There is also a “rescue” operation that takes a similar list, but the rescued values are left on the belt in original belt order, not in list order. Obviously you can use conform to get the effect of rescue; rescue exists because it has a more compact encoding, for use when you don’t care what the order is so long as they don’t fall off.
Any time you have a control-flow join point in which the in-bound arcs do not have congruent belts (such as at the top of a loop) then you pick one of the arcs, define its belt as canonical, and use conform on the other arcs to make everything congruent. For loops you generally want to define the most frequently taken backwards branch as the canonical one; that eliminates the conform that would be most frequently executed.