I really never came across this rule before. I've upgraded friends several times with my 500s, but now that I think of it, I think it was always at the airport. Often when I take a friend to the airport, I'll walk up to the counter with them and request to upgrade them with my 500 certs. Once I even signed a bluey for it... So I just assumed that they were transferable. Was this just check-in agents bending the rules for me each time? Looks like I'll have to split the record and just ask for an additional upgrade at check-in, or give away my F seat to my friend.