Will be flying round-trip EWR-NAS in less than two weeks time. We are in paid Z for the return leg but in S on the outbound. I'm considering upgrading us on the outbound flight as well, but I can't tell if it's actually available. When I go to the upgrade page, I'm offered upgrades for the 3 of us for 25k miles + $125 copay/person (Don't have that many miles available, so it would be out of the question) but under the buy up tab, the button is greyed out "No upgrade available" and under the details "No online upgrade available" Would it be worth calling UA to see if this is a website glitch? It seems odd that the seats would be immediately available for a mileage/copay upgrade but not for a direct buy up (after all, the website even claims there are "no capacity restrictions" and that you can "buy any available seats" with a paid upgrade).

It's not a glitch, it's a fare class availability issue. The up fare is not available. There may or may not be R space available for a miles upgrade. If there isn't, they will happily take your miles + copay and put you on the waitlist.