I'm going to do something similar, to but OTU points and then apply them to an existing reservation made within 7 months. Member Services explained that if not making a new reservation that I will need to make a "change" to my existing reservation and at that point I could modify the reservation in some way (dates, etc) and apply the OTU points.
Internally I disagreed with this assessment because procedurally a "modification" can be pushed through without any changes, and I do this when needing to reallocate points across reservations. But I accept I could be wrong in this specific example with OTU points so I listened to recommendation and booked an extra night and will play the game as recommended and, after buying OTU points, will modify down to remove one night to apply them. I'll then have extra "current use year" points that can then be banked.
There's a caveat if you used home resort priority to make your original booking that I don't think you can then replace with OTU points, but I may be mistaken here.