Apologies in advance for this n00b question...
As a 2P this year, I can select E+ seats at the time of booking. Since this benefit goes away next year, I'm thinking of crediting subsequent flights this year to another *A program (probably Asiana or Aegean). I've already qualified for 2P next year and unlikely to make 1P; but might make it to *A Gold on Asiana or Aegean next year.

My question is, now do I keep an E+ seat but credit the flight to another *A program?

Sorry, but you will lose your E+ seat. Because you will also lose UDU, you may want to consider buying E+ annual if you are switching programs out of MP.

wait till you have your BP and then change accounts.

Won't work on UA anymore. As soon as you switch from MP to some other *A #, if you are in E+, you lose the E+ seat unless you paid for the single trip or annually. UA has cracked down pretty hard on GA who leave you in E+ without a stated reason. The risk of doing this at the last minute is that the remaining seat(s) may be E- middles next to the lavs.

What if you use a kiosk? Would the computer automatically kick you out of E+ when the FF number is changed?

LAX

That's why you wait until you have your BP -- have not heard of anyone being dinged once they have their BP.

I am not speaking from experience, but I agree that it makes sense to just have it changed once you have your boarding pass. Most flights go out with agents having to fill E+ with non-elite passengers anyway, so I really doubt it would be a problem.

I watched this happen at LAX 2 weeks ago. I was painfully near the podium and pax got quite heated so couldn't avoid being subjected to same abuse as GA. She changed the # for the pax and then told him that he had a choice of window/middle in E- because E+ was wiped out unless he wanted to pay for it. She told him to go think about it and then called the flight so I've no idea what the end result was.

Rock bottom is that if there are people waiting for E+ who are due it for having paid annual fee or are elite and qualify freebie, a good GA will look for those who've gamed the system and transfer those who still qualify.

Can't have it both ways: miles on another carrier and freeloader in E+. Just try asking US for a Preferred seat as a UA *G!