Sheesh. This thread got tetchy fast!
I'll try to buck the trend by just providing my experience, with minimum commentary. My experience agrees with fastair's statements: rebooking on an AC codeshare is not only possible but has been routine for me. Once on PMUA as a lowly Premier (VDB -- I asked to be rebooked in J on AC and got it without argument) and more recently as a 1K flying out of YYZ (WX delay on my first leg was going to make me miss my connection).
In the latter case, the phone agent actually conferenced me in on her call with AC to get me the last seat on the AC flight (AC had Y availability, but no codeshare seats available). It was tricky, but I never felt that UA was balking at getting me where I needed to go, when I needed to get there, regardless of what it took.
FWIW, I've also been rebooked proactively on AS, AA, and even got protected on AA from ORD-PEK proactively (on request). The only thing I've never been able to get is a rebooking on WN. However, I suspect those positive experiences had a lot to do with 1K status.
Summary: it's totally possible, even routine, if you have high status on UA. If not, YMMV.