Ah, I see. It doesn't help you now but I believe one way to avoid this in the future would be to completely cancel the original reservation, confirm that the FNCs and points are returned to your account, and make a new reservation. You shouldn't have to do it like this but - as your experience shows - Marriott's IT can be a bit random so not giving it the opportunity to foul up what you expect it to do by manually simplifying the process can be the best strategy.