Just wanted to share an experiment I tried.

I used my United MP# to get Star Gold privelidges on a US Airways flight last week. AFTER boarding, sitting in my seat on the aircraft, I used my phone to change my FF# on the reservation to my US Airways number. I was just curious how it would credit. I assumed it was too late to change it, especially because the BP was scanned, but it ended up crediting to US Airways! I never had my US number on the reservation before that time.

Of course nothing is certain and maybe it depends on other factors (this was a US Express flight operated by Republic), but thought it was interesting.