When you move a Partner from Program A to Program B, all stats move with the Partner. That means if you move from Program B back to Program A, all stats are still there. The only thing that does change is which Subcollection the Partner appears in.
Also, if you change a Contact's Partner Program from Program A to Program B, Program A no longer shows him/her in the Partners Menu > Programs > Program A Subcollection because they are now in the Partners Menu > Programs > Program B Subcollection.
As for any open orders (payment plans/recurring subscriptions) at the contact level, those should remain the same as long as both the Product exists and the Partner exists. If the Partner is removed, all upcoming open orders will not be credited to that Partner because they were deleted.