Did a quick search but couldn't find anything on this. Apologies if I missed it.

It's been a while since I've redeemed UA miles. Trying to burn through the 50,000 balance I've got left. Just booked a trip from STL to SFO. Outbound has a connection through IAH. Used miles to upgrade the IAH-SFO segment. During the booking stage it allowed me to select First class seat. Not until after taking my credit card and confirming my reservation did it note that "Advance seat assignments are restricted for this flight. Please select a seat during check-in."

So am I to take it that I've got a confirmed first class seat on this flight or not? Will calling make any difference?

Thanks for any and all advice.