Yes, the alphanumeric username is ABC99999; the long MP account number is all digits. So thanks for sorting out the terminology.

However, the confusion is this: the United website yesterday would NOT ACCEPT the "new" alphanumeric account number--it would only accept the "old" all-numeral account number. There was a message "Please enter only letters A-Z or numbers 0-9"--ie, the mixture of letters/numbers in the "new" account numbers would not work, although it did work on 9/26.