United Bug Currently Blocking Upgrade Availability Online
Update 2/4: PZ upgrade space once again appears on United's website and app.
Confirmed upgrade availability may be tricky to come by, but at least United makes it easy to see whether or not the space is there, using a tool called "Expert Mode."
All of United's general customers and almost all elite members — up to and including Premier 1K — need availability in the "PZ" fare class to confirm an upgrade at booking. For one reason or another, PZ currently isn't showing up on any of the airline's public-facing platforms, including the website and mobile app. The PN fare class is still being displayed, however, so Global Services members remain unaffected.
Take, for example, United's nonstop from Newark (EWR) to Tokyo (NRT). Thursday's flight has at least nine seats open in every single fare class, except PZ. There's even 9+ seats available for saver awards in Polaris business class, but not a single open for upgrades. That doesn't make any sense.
That's also the case when you search using the airline's upgrade tool. According to United's website, Premier 1K upgrade certificates will land on the waitlist, rather than confirming right away.
Fortunately — as many have likely expected — the airline isn't suddenly blocking upgrades on every flight, it's just a bug with the website and app, and hopefully one that will be corrected very soon. United is aware and looking into getting it sorted ASAP. In the meantime, you can check upgrade availability before you book by giving United a call at 1 (800) 864-8331, or via your respective elite line.
If you prefer to search online, keep an eye out for IN availability. If you even see a seat for saver awards, there should be at least one open in PZ. Once you confirm it's there, you can still book and upgrade your flight online. If for some reason it doesn't clear, be sure to give United a ring right away.