Koji has it pretty much right. If you get evacuated, either because the post is closing or on an individual level for, say, medical reasons, there should be no problem re-applying and getting accepted. In fact, the Post Staff will usually put this to you to expedite the process.
The long answer:
Specific to post evacuations, the PCVs who are close to COS are usually just granted COS, while newer PCVs are initially given new assignments. I know in the past they have let these PCVs choose where they wish to go, and they either finish out their remaining time or are but back through PST and a full new 27 months. I do believe they also have the option of taking the COS and walking away, with full ability to reapply at a later date.
I've noticed a ton of confusion about the terminology around leaving early. Most people seem to think an Early Termination is a bad thing. In reality, anything that involves a Volunteer leaving early is an ET (except a full-post shut, or a medical or admin separation). If you want to go home early, you can 'Resign,' which is considered an ET and is generally not looked on favorably, especially for re-applying (although this DOES NOT exclude you from reapplying [only admin sep does], you will need to account for it with your new placement officer).
This mostly comes down to the Country Director; they're the only ones who can offer a Close of Service prior to your original COS date. If you're trying to move it up more than 90 days (? might be 30), your CD has to get approval from the Regional Director. In general, CDs are not willy-nilly with giving out advanced COS's; you need a justified resign why you don't wish to just resign. Because of this, it isn't that hard for a CD to get approval from the Region Director, although it still is not commonplace.
Medical Separations are more common, and depending on the condition this may preclude you from future service (although legally they cannot make this a hard and fast rule). I know med seps are pretty common if you have a physical or emotional issue that is challenge your service and you wish to go home. I knew a volunteer who was sexually assaulted, took a medical separation, and eventually re-applied and was invited no problem. Only the Medical Officers can grant this, and it is fully up to their discretion. A medical separation cannot be challenge by any other staff.