You do have to be a little careful when adding your dining plan after the fact. Such as, if you already have a specific room type in mind reserved, and add or upgrade a dining plan on your account, they have to cancel your already existing reservation on your room, and reopen a different one. When this happens, they have to refund the total amount you have already paid to your credit card, and will expect payment again. At least a $200 down payment to make the reservation.
We just went through this with having a river view room reserved at POFQ already booked and a week later added a QS dining plan. They then had to cancel our reservation and start a new one. We had to wait a few days to get our money back into the account before we could pay for the new room and dining plan. We did end up keeping a river view room though. Then, a few weeks later, we upgraded to the TS dining plan and had to go through the whole process of canceling and reopening again. However, this time, we lost our river view room because they had booked all of them by the time we were able to get back into the system. However, through all of this, every one of our FP+ and ADR's stayed right where they were, just the room reservation was changed. We'll be heading down there in a few weeks, and I'm sure by the time we get there I won't care anymore. But, it was definitely a big stressful hassle that caused a bit of bickering between the DW and I. I was told by a CM to complain to a manager and they would make it all better. But, I have yet to do so.