So I'll be checking a bag through ATH-LHR-EWR. It's one J mixed-cabin ticket booked with UA miles - first leg economy operated with Aegean, 2nd leg Polaris United.

I know the bag will get checked all the way through with no problems, but I'm wondering on the bag rules when starting with an economy ticket. Will I have to pay a bag fee at the Aegean counter, since the ticket starts as economy? Would the bag somehow get tagged as Priority along the way?