What Dean said. If you have enough points, that's the way to do it -- the safe way.
If you cancel first, you or the MS CM might see 1 BR availability but by the time you cancel one ressie and then go to book it, it might be gone. Things do move quickly.
There have even been situations where a member canceled, was unable to book the desired ressie they saw only moments before, and then was also unable to get their original ressie back because it had been snatched up immediately.