With Starwood, any time you make a reservation that is different than what you own, it is a Staroption reservation. So to make a reservation for a different season, a different unit, or a different resort, you use your Staroptions.
The earliest you can make a Staroption reservation is 8 mos. before check-in.
When you reserve exactly what you own - it is a home resort reservation - the earliest you can make a home resort reservation is 12 mos. before check-in. The latest you can make a home resort reservation is 8 mos. and one day before check-in.
However, as sptung pointed out, with the unit you are considering, you are not going to have very many Staroptions, so using just one side of the unit may not be enough Staroptions for the trades you want. It completely depends on where you want to go, the season, and the size unit.