Tigger2on, What is happening is that even though it is a "new" baby, it is still another person, accordance with the marine laws. The room that you have is specifically for 3 people. It is that another cat 11 handicapped room for 4 people not available??
I would do a wait list. I would also insist what other handicapped room are available currently...After all, they do upgrade guests from time to time, i would thought Disney would be more accomodating to your situation.
Now, i know that the others are offering to put the infant in a another guest name/stateroom, which sounds like a great idea, but here is the danger....
if the room specifically fits 3 people, it should only be 3 people, that is not a Disney Rule, but a Marine law. I have a cat 6 room and my mother has a cat 11 and my kids are in the cat 11 with my mom so i can pay cheaper for the kids, but both staterooms specifally sleep 4, so i can have my 2 kids go back and forth to each of the staterooms. If the 'housekeeping' knows there is a 4th person in the room even though it is just a "little baby and does not take up room, don't need an upper bunk, etc..." but accordance with Marine Law, it is "a person", this could get into a messy trouble and i hate to see your vacation ruined over this.
Hygeine99 stated that once you are on board, they don't care where you stay....not necessarly true, they don't care if the stateroom provide the number of true people in the room. But word will get out, if the baby was moved into your room, which is specifically for 3 people, for the whole 2 weeks permantly, at least by the stateroom host. ( unless, he/she just doesn't say anything, which is by chance)
The last time we cruised, my kids was going back and forth and our stateroom host, told us he didn't care as long as there were correct number of bedding. ( He even questioned when he thought we had an 3 kids in our room, thinking we had 5 people instead of 4!!) It not like a hotel room that you may be able to get away with. It will also caused alot of confusing with regarding, the marine drill, dining, since the baby will be under another stateroom. Suspiusion will rise...
I would work harder and get another handicapped room/cat 11 that sleep 4 at the same rate that you paid for at the original ressies, (unless it is booked, most likely) but pay a few hundred more to cover the porting fees for the baby.
I will call myself if you need me too and see if i get a different answer. I will be glad to help you any way i can...
Good luck!!!