I have a connecting itinerary in T and only the second leg cleared CPU. For some reason, the SDC page in the app didn't offer any option at all (pretty sure it's been broken a a few weeks if any upgrade has cleared) even though T is available on a few flights. I then purchased day of departure upgrade on the first leg and I was booked into P.
I tried to SDC to a later flight in the evening but the app still shows nothing. Then I tried calling and the agent said I could only change to T and have the upgrade fee refunded, which makes sense per the rules but I was also worried that the upgrade fee is nonrefundable.
So I tried the change flight page myself and several flights under the economy column have +0 fare difference but have V listed under it and +$200-300 under first class column in P class. I clicked on one of the +0 ones but at the checkout page, it said I would book into "United Economy (P)" with no fare difference. Did that and I successfully changed into a non-stop in P (supposedly "United Economy").
If IT is watching this, this is an expected behavior and no code change is needed