I've actually asked about the upgrade process and here's what the fine folks at MVPG Customer Care told me:

Let's say there are two PAX going for one remaining F seat. If one has MVPG and the other is MVP, MVPG gets it. If both PAX are MVPG, then the tie-breaker would be ticket purchase price. If both PAX purchased their tickets at the same price, then the PAX that purchased their ticket first gets the seat. Don't know what happens if they both purchased same date...probably shouldn't get that far!