My only experience with this was a rather odd one:
Flight AMS-ORD with Y fare upgraded to Business (pre BF branding days). The flight then goes mechanical.

I'm rebooked back in Y on a flight to IAD with no connecting flight to get me to my actual destination. The same AMS-ORD flight is running as an extra section in addition to the normal flight that day, so if anything there should be more business seats than fewer, as surely some passengers were reaccommodated in other ways.

I call and get stonewalled by the agent who doesn't want to put me back in business on the new section. I thankfully get a supervisor who very helpfully rebooks my entire ticket in full C for the business segment and full F for the domestic connection (and I got the mileage bonuses).

Just before boarding, I'm paged to the desk at the gate. I think I've been foiled, but the station manager hands me a boarding pass with 1A on it. So, in the end I got a double upgrade.

Oh, and to connect to the original thread, they said that anyone with upgrades could not be accommodated in business on another carrier...worked out to stick with UA.