I felt checking in early or not has not got much to do with op up chances at all. CX has rules about op ups, some times when lower classes were heavily oversold, they start op up high tier elite flyers very early and we could even select upgraded seats at olci.
I personally felt most op-ups happens around -3 hours and normally it wouldn't matter if we've been assigned seats or not.
And when flights were not that oversold, it seems CX might need to wait until the last few pax checked in to actually op-up someone, and obviously again, it wouldn't matter if the pax selected been assigned seats or not