For some reason Disney seems to have a problem with modifying an existing reservation. When some one calls to make a change they will actually make a totaly new reservation and cancel your old reservation. Since your reservation is on the BRP you are allowed only 1 room reserved, so when the CM enters your new reservation you get kicked out of the BRP since you already have a room reserved. Do you have another AP you can use for your new reservation? But now you get into the non-refundable deposit in your current reservation. Oh well, good luck.
P.S. You will have to have all this worked out before you hit 60 days before arrival or you won't qualify for BRP at all.