I flew from IAH-CHS with a guy who came from KUL. I didn't ask about his exact schedule, but his arrival airport to the US was SFO.

He said they were scheduled on a 787 and it went mechanical, so in order to prevent a huge delay or cancel, UA borrowed a 747 from Air China and they flew that plane instead. He wasn't sure on the pilots, but the crew he said was UA.

Can this happen?
Is there any way to tell?

Maybe we were both drunk and I mis heard him....