I tried to eUp daughter at airport. Forgot it was a Tango fare I booked online. If you bought direct from AC, you can likely do that. If through TA, not sure.
(Actually, I did not even ask, but when explaining why eUp was not available, the agent proactively offered me to pay the Tango-Flex difference on the day of travel, so I could, but it wasn't worth the $ to me. Moved her into my vacated Preferred Y seat and I sat in J.)