Yeah, I don't recall the state of being checked-in on a UA flight causing issues inhibiting SDC or change/cancel. In fact I don't recall there being a separate step to "undo check-in", at least as part of an SDC. It just happened when confirming the change.

I do remember it causing issues on other carriers like AA where being checked on a partially cleared upgrade caused all sorts of problems when only one segment that was part of an instrument upgrade cleared and they had problems re-ticketing when the other segment cleared at some point after I checked-in. But that's not too surprising since AAs IT is in shambles compared with UA.