I have struggled a lot trying to upgrade Austrian KEF-VIE, called hotline several times, in Poland and Germany. Each time rejected despite there were free seats. Two agents in Germany even won't try as they've said it's not possible to do it for two pax (having 1 eV each), this really puzzled me...

Today I have decided to try another booking, with LOT, for end of April. There was instant upgrade confirmed for both of us. Agent was even able to tell me just before submitting, that it will be instantly confirmed.

So it seems to be dependant on the airline, looking especially problematic in LHG, at least in my case.

BTW, this thread is super helpful!