Yes, you can. The 800-number is on your exchange certificate. You'll need the Disney reservation numbers, which are also on your exchange certificates.
Note that some things can be requested, while others cannot. If something is a "booking category" with tracked inventory, then the week that was deposited already determines that---it was taken out of specific inventory. For example, dedicated vs. lock-off status is a booking category, not a request.
I have reservations for March 2009 and I just called yesterday and added the DDP and requested floor & view preference.
The 800# was not on my interval confirmation and I spent a long time on the phone trying to get the right people to help me. If you don't have it either the number is 800-800-9800. Then press 1 and 3. Make sure you press the option for "exchange".
The telephone number to call Disney Member Services is on the second page - 2nd paragraph under Important Information. They need the reservation number which is found on the 1st page - upper right corner - under the bedroom count.
You can make requests for room location, things you may need for the room (real crib (not pack and play), high chair, bed rails are some of the ones I have done. You can also add the dining plan and magical express. They will also help you with your Advanced Dining Reservations to!