I searched the forum but could not find anything relevant - pardon me if it's been asked before.
I was told that you need A or C fare class space available for the SWU to clear instantly, but if there's only A fare class available will it upgrade me to biz from economy on a 3 cabin? Or does it have to be C for economy to biz (or to first on a 2 cabin) and A will work only from biz to first?