As long as there is availability, you can switch out the reservation. In many cases, they can do it without much risk.
However, in certain cases, they have to cancel the old before making the new and in those minutes, there is always the chance that you could lose your original reservation.
This happened to me just last week. They had a BLT room I wanted but I lost my BWV room in the process. She told me there was a chance but was also showing 4 BWV rooms like I had so we figured they wouldn't go in 30 seconds. Well they did and I lost it.
I put myself on a WL for the original room and within 30 minutes, I had the room back. I think that the first CM I talked to didn't know what she was doing and that is why I lost it, but that sometimes happens.
I have also heard of people asking MS to make the new reservation (if they had the extra points) before canceling the old one.
Good luck!