I concur with Goldorak. I saw that once in a DXB-CDG where I was flying in PE: purser went to 2-3 pax with a tablet in her hand, discussed with them for a couple of minutes and they moved to C. I guess they paid by miles because I have not seen any credit card used.

On a separate occasion, on the same flight I approached the steward once boarding was completed and asked about a possible upgrade, he said that it was indeed possible in theory but on that day they were "technical issues" and could not offer any upgrade.