You can have the onboard agent contact DU via email to release the deposit on their existing ressie and somehow apply the OBC & 10% to the reservation and keep your original room. Kcashner explained how it happened when she did this and it was easier than ever before.
Also, a few years ago I had our OBB with DU and late one Saturday night when the TA could not have called
DCL a room came up that I wanted to transfer to. I emailed Tammy and she got right back to me and explained that I needed to book the room we wanted now, send her the ressie number and she would handle combining the reservations to keep main dining, the original price [which had gone up] and get the room I wanted. I got an email back before 9 am Monday morning with the confirmation that it had been done.