If your upgrade clears prior to check-in, you can go into AC's Manage my Bookings and swap out your AE number for that of another FFP. In fact, I did that yesterday, and I still have my seat in J.

If your upgrade does not clear prior to check-in and you must play gate lotto, you cannot change your AE number as I believe it is required to be placed on the upgrade waitlist when using e-Ups. If your upgrade clears at the gate, you can technically change your FFN to that of another program. In reality, you may encounter some resistance from the GA/other AC personnel who refuse to change your FFN/claim it cannot be done/claim that you will lose your J seat.