You'll have to ask the owners whether the unit is Phase One (Virgin Grand aka Hillside Villas) or Phase Two (Bay Vista). If it's Phase Two, and it's a "pure float," the early bird gets the worm -- call on the opening bell exactly 12 months in advance and you'll likely get the week you want. If it's Phase One, it's essentially fixed to the deeded week and although you CAN trade to other weeks in the gold season, it's based on availability .... reportedly very difficult .... definitely wouldn't count on it.
The system works the same way for Phase One units at both WSJ and Harborside. Therefore, it's pretty important to buy a week that works with your schedule and desired vacation time.