I flew JFK-CUN in 2012 and was upgraded in both directions.
On the outbound, when the system tried to EUA me, it somehow messed up the ticket. Lengthy phone calls with Delta fixed the ticket but not the EUA. At check-in, the agent was fabulous - she understood the EUA process and upon showing my PM card upgraded me.
I don't remember whether on the return flight I got the EUA or if I had to talk to someone at check in, but if the latter, it was similarly quick.