It's been a while since I've done OLCI on a PNR with more than one person. If I recall, there's specifically a question asking if you want to split the PNR (ie. preserve the upgrade or be removed). If I understand you correctly, if you choose the preserve the upgrade (ie. split), it does not actually do the split unless the upgrade clears? When/if that happens, the other pax (ie. no status) would retain the upgraded pax's status?

I just seem to remember one time in the past, doing this and it actually splitting, causing issues with the other pax trying to check a bag. I distinctly remember that no one got upgraded though. This was a while ago though, perhaps back in the auto-checkin era.