Thanks, your suggestion to get AC to waive the bag fee by showing the AC 25K status is great, and will be the best solution if it works.
However, if it doesn’t work then I guess the backup plan is to change the FF number (or pay the bag fee, which is obviously what I want to avoid).
I’m not too worried about where the miles will be credited, and also regarding lounge access I’m sure AC/UA lounges allow *G access so no concerns about that either.
So coming back to the backup plan in case AC won’t just waive the bag fee, I guess I will ask them to change the FF number, and then optionally ask to change it back to UA at EWR, or maybe at check-in for the return trip. Right ?