Thanks, MisterNice and everyone else.
I did call the Elite line this morning, and the agent said I had an upgrade but not a seat assignment - there were plenty of seats but only one aisle seat left (my preference) so I got that. She apologized for the glitch and didn't know why it happened. She did say that she had heard of this happening sometimes when it's the last upgrade seat, but that wasn't the case here as First Class (on a 738) still had 8 or 9 seats. Oh well...looking forward to EUA #2.