There's a lot of restrictions on use of this option - you have to have the pass at the time of booking and flying and credit miles to UA's Mileage Plus, so before booking this, I'd call UA and ask if the Annual Option would cover your trip - it should but call to confirm that. (if you had flown a lot and gotten Premier Status on UA, then you'd be eligible to use E+ as long as you had Premier status at the time of flying, while the Annual Pass says it's more restrictive, you need to have the pass at the time of booking AND the time of flying. I think the restriction means at the time of booking the E+ seat, rather than booking the flight, but best to check on this.)

So can I book a flight on UA using my MP number (no status), pay for E+ and then before flight time, change the MP # to my One Pass number and still retain my E+ seats and get credit for the flights?