I was re-routed from LHR-EWR-IAH on UA to BA LHR-IAH nonstop with storms at Heathrow a few weeks ago. I had actually switched from UA to AC LHR-YYZ-IAH on the phone, and then the UA checkin agents at LHR were able to give me the BA nonstop when I arrived at the airport. All UA ticket stock.

ORC was easy. I had to call the 1K line after radio silence over email, and the agent handled the ORC over the phone. Still fighting for AA miles (am double dipping with ORC on UA and crediting the BA J ticket to AA) but AA is confused by the BA flight on 016 ticket stock, I think.