Guess I’ m a bit confused how this usually works. Every website told me to check upgrade availability before booking the economy ticket. I did and ExpertFlyer showed 4 J seats available.
But now I’m not able to book them using my United miles... is there a way to check upgrade availability on United before booking the fare? It seems like a confirmation / ticket number is required...
There us no way to guarantee this. So what is commonly done is to book the ticket with a 24 hr cancellation option and looking to confirm the upgrade. If that does not work then cancel the ticket. Rinse and repeat as needed.
There are a few where you know it will work. Like I/O available on NH essentially guarantees the upgrade.