I made exactly this transition a few years ago (UA MP to BAEC). UK-ORD is one of of popular routes. If you can get BA WTP signed off as normal, then I'd make the switch and don't look back. It's simply better than UA E+ (or AA MCE). You'll earn enough Avios to upgrade a reasonable proportion of the flights to CW (if you can find availability - which is not too bad on the LHR-ORD route).