Many years ago (c. 2000) a very friendly UA phone agent bumped my J seat to F. Another agent caught it a few weeks later, and proactively downgraded me back to J. This sort of thing was known to happen from time to time, and it was also known that you were always subject to downgrade, particularly by the Global First lounge staff at SFO, who would proactively audit reservations on their own initiative. For this reason, the standard advice on this forum was "don't go to the SFO GFL if your upgrade won't survive an audit."

Bottom line is that an unsupported UA upgrade is always subject to downgrade, and it is wise not to ever give any UA agent reason to take a closer look at the reservation.