I booked the TACA LAX-LIM deal earlier this year before TA joined *A, and since I didn't know it was about to happen, I entered my LifeMiles number. Last month, when checking in online for the outbound, I replaced the LM number with my MileagePlus number. The MP number is on the outbound boarding passes (including the UA prefix). Similarly, when checking in for the return, I again replaced the LM number with the MP number. I didn't notice until a few days ago that the return BPs show the LM number despite my change.

Last week when I was wondering why miles have not yet arrived in my MP account, it occurred to me to check the LM account, and don'tcha know - they credited all flights to LM, including the two segments whose BPs show the MP number.

Shot an email to TA asking them to remove from LM and credit to MP, but got a reply that they will not do that. Oh well, guess I can redeem for a lounge pass.

I wonder if changing the FF assignment at the airports rather than online would have worked better than on their website.