The way in which the "old" Marriott Rewards used to work is you would have been easily able to lop off the additional nights -- although you likely would have received re-pricing as you would no longer qualify for the 5th night free.

In any event, the "new" Marriott Rewards does not work this way -- it re-prices the room as of the new booking date, and if the room/room type is not available, you are likely SOL.

Margarita Girl had success likely because she not only had a competent agent, but also because there was award space still available; albeit at a slightly higher points rate.

Not sure if the same will obtain for you, unfortunately, however, the suggestions above are a good starting point.

I always now book 1 night reward reservations so I can easily lop them off if they are not needed, as I rarely if ever have used the 5th night free benefit.

Good luck!.