A lot of these ressie swaps get done all the time, but they can go bad. Sometimes you check availability, it's there, so you tell MS to cancel the old ressie and book the new.
They cancel, but when they try to book the new dates, the availability has disappeared while they were cancelling the original ressie! If you are visiting at a peak time, you might even run into the situation where the old canceled reservation is no longer available either!
This has never happened to us, but I've seen a number of threads where it happened to others.
If you have enough available points to do so without creating any issues in your account, you might make the new reservation first and then cancel the old ressie. That's the only way to really protect yourself.