Hmmm, I wonder.

The process is like (1) make a reservation in V, (2) issue a ticket, usually a V fare basis, which the airline accepts to cover that reservation. Then click the 'upgrade' button in the app, which causes United to (3) change the booking class on the sector in the reservation from 'V' to 'R', then (4) reissue the ticket with fare class 'R' on the segment in question. Between steps (3) and (4) there may be a fair amount of time, and sometimes you may call UA and force them to reissue the ticket — "hi, my upgrade cleared and my reservation shows an angry red message that says 'A modification has been made to your itinerary. Please contact United Reservations to have your ticket reissued.'"

Since the V=>R transition only happens on UA metal, UA can just revalidate the ticket (change their internal info on the ticket so that the segment has a new fare class) and they often do not need to issue a new e-ticket number. Instead they 'revalidate' the existing ticket. If you use united.com "update e-mail address and send receipt" they will send you an e-ticket receipt which shows a ticket with the sector in "R" class. If you look up an e-ticket after revalidation using a third party, like the saudiairlines site which shows the OAL view of an e-ticket, you will probably still see "V" — after a ticket is revalidated, the new ticket info is not sent to other carriers.

But I think UA does reissue a ticket that says "R" when they process an upgrade. This ticket reissue can be the point where they process the add/collect for an upgrade fee or Air Passenger Duty (it's weird that they do this in advance when waitlisting). I'll have to look and see whether I have any "R" class tickets which have subsequently been reissued with a new eticket number where it might be possible to dig up the third-party saudiairlines style view of the eticket and see "R".