Forced Upgrade Scenario
FO has no proffer in place, but does have displacement preference of OCL. Company makes the FO upgrade and since no proffer, they displace them to OCL, assuming of course they can hold in with their seniority. You are junior to this pilot and happen to be the very bottom OCL on the list. You could get bumped out of base.

Normal Vacancy Scenario
FO has a proffer in place for OCL and is senior to you. You will not get displaced out of that base just because they come in, since in this case they are entering with a vacancy bid, essentially meaning there are open slots.

So while you can get displaced out of a base due to the first scenario, it is this exact scenario that will allow you to get back to it. If you get displaced out, you are not locked, so you can proffer back to it once your seniority can hold it.

It would suck yes, but you will be able to get back.