"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?"

As a CO Platinum, you should be able to book a UA flight, NOT pay for E+, enter you CO FF #, and call a friendly agent and ask them to put you E+. You may have to wait until the day of travel, or you may be able to do this in advance.

I was able to do the reverse, i.e., book a CO flight, w/ my UA MP #, then call and get Preferred seats over a month in advance of the trip.

If UA is being difficult and won't do the same (E+ access w/ CO number in record), try again a few minutes later. It seems you should always have the option of buying E+ access on a per flight basis.

The safest way would probably be to book the UA flight and try for E+ access w/ your CO number in the record and cancel w/in 24 hours if it doesn't work, then try again another day.

If you are required to have your MP# in record for E+ access, think you can switch it a few days before flight and you're unlikely to be booted out of E+ by the robot, should the robot be taking a harsh line on E+ access.

All of this should be much simpler Oct. 1... hope so