Since the reservation is on a different membership, the only way to use points from the other one is to transfer. The points transferred can not already be banked or borrowed.
In addition, if the membership in which the trip was booked, has banked or borrowed points, then those have to be used before transferred points can be used
The best option is to try and rebook nights from the correct membership, beginning or ending dates, and then reduce the original one. They can then link if all nights are not gotten. At least that would save some.
Another option, which is risky, is to cancel one night at a time snd hope there is no waitlist and pick up that way.