While I know you're speaking generally, CO is not the carrier I'd use as an example of a proven track record with this. Their EUA system is sporadic at best, and CO's sweep methodology leaves much to be desired.

I'm assuming UA's system will list people, much as it does today for e500's. This is how DL's system works, and it's much, much cleaner. If a seat opens up, it goes to the first on the list. With CO's system, if a seat opens up, it goes to the first full-fare Elite who calls, or the first person on the list, if it's still there when their algorithm gets around to checking that flight, which could be several hours away.