After some back and forth, the UA social team helped me out and honored the price quoted online. It required screencaps showing the online price.
I am not so sure the ticket stock is the issue because they had no problem processing the upgrade by phone. It was only by paying online that was the issue. Appears to be a website bug more than anything else.
Last edited by F 1; Mar 21, 2019 at 3:20 pm
Reason: typo