That won't work now (that is, if you redeemed UA miles for that ticket). UA's system has a record of that ticket being an award ticket, and any changes down the line affecting the fare bucket won't result in any credit on UA. This happened to me. I was flying on a UA award ticket with the final segment being a TATL on Lufthansa. I was able to buy an upgrade to Premium Economy at the gate, and my ticket was "resissued" to E (but retained the original UA ticket number). If I had thought of it at the time, I probably could have switched my FF# to M&M and gotten some sort of credit for that segment.