Out of curiosity... has anyone repriced an itinerary that they've previously upgraded using miles?

I've purchased a K-class ticket in December and used miles for BF upgrade, if a fare class opens up later that makes it worthwhile to reprice/reticket, would that even be possible? Assuming F class was no longer available, would I lose the upgrade (as I suspect)? Or would they let me keep it and charge the difference in the service fee for the new fare class?