You may do that. The potential issues are you might 1. have to pay a prevailing price at the time or cancelling and rebooking, 2. lose OBC (either from a. Celebrity if they don't have it anymore or if the current OBC is less than from the original booking or b. your TA if TA's OBC was linked to an agency's own promotion, for example, and it doesn't exist anymore), and 3. current tax might be higher, too.