Modifications can be made when a promo has expired as long as what you want is available under the promo. With that said, it was never a problem to take nights off, but Disney recently changed the policy. The shorter stay must now be available in order to take nights off without losing the discount. If the new nights are available under FD, you can modify to the shorter stay; if the new nights are not available under FD, you can't shorten the stay without losing FD. Also, the new arrival date must have been included in the dates of the promo. If you were originally checking in on the last (or second to last) day of FD but will now arrive later, the new dates wouldn't qualify for FD. As the pp said, since the promo has expired, you will need to speak to someone in guest services as CRO doesn't have the ability to check availability/modify a reservation with an expired promo.