Reviving this thread, I am still stuck. I am the person with miles, trying to book for someone else. I ran into the same problem that my information is locked in as the first passenger and can't be changed. I have successfully booked the tickets using the other person's account, but I can't figure out how to pull up the PNR and pay for it when I am signed in with my account.

I'm waiting for a call back from AA now -- 4 hour plus wait!