I have a domestic roundtrip booked. I might not take the return flight. I'm considering calling UA to cancel the return flight on my itin, partly because I may take a different flight that's earlier than the originally scheduled return flight, and I don't want to be snagged by any double-booking / back-to-back scans. [Edit to make it clearer: the "different flight" I might take instead is to a completely different destination]

I know that technically, if I tell them I won't take the return flight, they should refare it as a one-way, plus a $100 change fee. Realistically though, will they actually want to do this (and make a note in my record etc), or would they usually just thank me for letting them resell the seat, cancel the return, and move merrily along?