Doesn't matter. Unless they've changed it, you re-bid according to your new status seniority for whatever is left over (or they assign you something or you carry over). It's why senior F/O's who get summer vacations can't become junior reserve captains and keep that senior vacation slot. Otherwise all the formerly senior F/O's who've kept their summer vacations would leave the company with no junior reserve captains during the heavy summer months when the senior captains are all on vacation and when people are calling in sick more often to boot.

It's the price of upgrade and the concept of seniority.