I have not been successful on many occasions on M fares doing the E upgrade window. The only thing I hope is that AC (with the exception of the concierge override for an SE) is following proper protocol for the pecking order of upgrades. Sometimes I have a sneaking suspicion that things aren't done properly, but I hope I'm wrong in this regard.

But having said that, I have been pleasantly surprised once NRT YYZ when I got my ug at the gate after going on the waitlist at the 96 hour window.