I will be travelling with some friends tomorrow who are on different PNRs and want to try to eUpgrade them

I understand that I can upgrade up to 2 as a travel companion.

Am I correct that this *must* be done at the airport? i.e., I can’t do it over the phone after check in?

If so, I would like to try to make it a surprise … do they need to be with me? Or is it enough that I have their PNRs?

Finally is there a way to preview the potential co-pay amount? On the AC website I can see their bucket - like K or whatever -, but not the brand. So it’s a bit unclear to me what/if I will need to pay.