It does not happen often that they do a 320->319 swap, but it does happen. The chances of it happening with a full F cabin is probably less. No one here will know for sure, but you will probably get a lot of anecdotes.

As a paid F pax, I would expect that you would be accomodated before any upgraders in the cabin, but perhaps they will just reseat those in row 3. It is difficult to know. Regardless, if you can't get your seat changed to row 1 or 2 then you should monitor the flight closely and if a change does occur you should call immediately and ask to be reseated in F.

If the swap does happen (and remember this isn't likely) you will be downgraded or offered accomodations on another flight. If you choose the downgrade, you will get the fare difference between your paid F fare and the full Y fare. If you are on a QUP/YUP, this might not be anything at all. You will also get a paper CR-1 upgrade cert and perhaps a $ off cert. If you don't get the latter, writing to CS will probably score you one.