I am in agreement with UncleDude.

C1 still may mean there is a waitlist of one PNR awaiting 3 upgrades. Upgrades do not clear one at a time on a single PNR.

My boss needed C3 (wife and daughter). There was C1 (no one cleared from his PNR) and when I saw it I called and grabbed the one upgrade (wife I assumed) and then a few weeks later the remaining C2 cleared.

So no, C1 would not mean that all upgrades are cleared. I guess however I would say C4 would mean it was much more likely that most upgrades had cleared (unless there was a party of 5 pax on a PNR still waiting).

If you see a C1 GRAB IT FAST!