Since someone already posted it elsewhere, I will just share it here, expect UA to bock this anonymous access soon.
You don't need to register any service to see the future upgrade list, if you are familiar with computer stuff a simple curl will do it, you can pair it with jq for your viewing preference.
curl -H "Host: smartphone.united.com" -H "Content-Type: application/json" -H "Accept: */*" -H "User-Agent: UnitedCustomerFacingIPhone/4.1.37.1 CFNetwork/1240.0.4 Darwin/20.6.0" -H "Accept-Language: en-ca" --compressed "https://smartphone.united.com/UnitedMobileDataServices/api/UpgradeList/GetUpgradeList?accessCode=ACCESSCODE&appversion=4. 1.37&applicationId=1&languageCode=en-US&departureAirportCode=EWR&transactionId=&flightN umber=14&flightDate=20220211"
Or simply replace the departure airport, flight number and date in the following url and look for the upgrade list towards the bottom. Now easy to read but will do the job.
https://smartphone.united.com/United...pplicationId=1
I created a small tool to construct the upgrade list url, put in your flight number, departure airport and date to the bottom right window of this link and submit.
https://jsfiddle.net/uvgdf5nq/