I have done this many times and never lost a reservation. As long as you booked within the 7 month window, I have never had a problem swapping out points. Just a word of warning - not all CM's are created equal. Make sure that you get a supervisor to help you, and if you get a bad vibe from them (what they are saying makes you think they haven't done this very often), hang up and call back.
The supervisor can hold onto your current reservation while the points are switched out. Make sure you do it before your 31 day window, or the cancelled points will go into a holding account.
I have points at 4 different home resorts, so I never know which points to use at the 7 month window. Usually, I pick the wrong ones - and need those points to use at the 11 month window - hence all the switching out!