I'm not sure about this Eireman, especially since EI stopped codesharing with AA on Transatlantic routes, switching to the alliance with UA.

I'm sure it would be possible on the day at MCO, as if there is still space available, an Upgrade to 'Premium Economy' is advertised at the check-in counters. The problem may be selecting such a seat in advance.

However, its definitely worth a phonecall to EI in Dublin or EI in Jericho, NY to check as to whether its possible.

Maybe if you were to book the LAX-MCO and MCO-DUB sectors separately?

EI Premier