Do I have to say yes to VDP to be able to alternate?

Affected employee.

My department will do a VDP for 30 days before doing the SERLO.

Do I have to say yes to VDP and become an opting employee (after those 30 days) to be able to alternate? Or can I do it now?

I would rather find an alternate now and be ahead of the curve, than wait later to not get a position from my current employer and have more colleagues looking for jobs.