United has relented and will now allow you to register to receive double elite miles even on previously purchased flights. This leaves AA and Delta as the only airlines not allowing tickets bought before the double elite miles promo to earn double elite miles.
-Say “Manage My Account”
-Enter Mileage Plus number
-Enter ZIP code
-Say “More Options”
-Say “Register for a promotion”
-Say “MPW999” or something like “M as in Mary, P as Paul, W as in W, the number 9, the number 9, the number 9”
-It will say “Double EQM Ticketing Exception MPW029”
-Say “yes” to confirm
-It will confirm registration.”
Thanks to this FT posting for the info!