I's Starpoints that you have to alternate. You can make Staroption exchanges every year if you wish.
Just to be clear, you don't have to alternate. If you only own one week in the Starwood system, you can convert your unit to StarPoints every other year. If you think of your Lockout unit as two separate units, you can alternate between them to convert each year.
Or you can choose to convert both units one year, and not convert any the next. Your choice.
If you are a multi-week owner, at Elite level II (or is it III, I forget.) or above, you can convert your units every year.