Clarification: Although all Hillside weeks were sold as fixed, owners are able to float ahead of the general SO population. If an owner doesn't "hold" his/her week by month 10, it opens up and can be booked by another WSJ Hillside owner (have to own same season) .... and at 8 months by all SO owners.
Per reports on this site, an owner can also give up his/her reservation at the 12-month mark ... and that too would only be open to other "same season" WSJ owners until the 8 month mark.
This is all possible -- but, it rarely works. Most owners book/hold their weeks ... they may try to get another week, but don't give up theirs until they get something (kind of a vicious circle!).
Just like with HRA Phase One, it's critical that you buy a week you can use.
Hope that helps!