Must comment.... in database terms, your Res Number for a vacation is referred to as your "primary key".
WDW probably had a real NEED to change these - most likely due to EXPANSION of reservations. But this is a very DANGEROUS action..... it can screw up past data beyond belief, and needs to be done by EXPERT staff.
So - keep an eye on your current data.... and be ready to CALL CALL CALL like mad. This sort of operation CAN be done successfully.... by really GOOD DataBase coders. Just commenting that really GOOD IT staff do not appear to exist at Disney - WATCH OUT! The possibility of "disconnected data" is fairly high.