My understanding is that if you do SDS from a FC ticket (either paid FC, paid upgrade, or confirmed GUC/RUC upgrade), you are put on the standby list for coach, You can't be added to the upgrade list until you are confirmed on the flight, which is extremely likely to happen only after the GA has cleared all upgrades for the flight.

This is for voluntary standby, not someone trying to be rebooked during IROPs or someone who (for whatever reason) is a confirmed FC passenger without a seat assignment. Also, in the hypothetical case where someone with a FC ticket (paid, paid upgrade, or confirmed GUC/RUC) can't do SDC onto the new flight in FC, but can SDC into a coach seat, the person would be confirmed on the flight so that this would be a DM on the upgrade list with a very high fare class (above all coach and C+ fare classes), but I think by the rules they would fall below a coach passenger trying to use a GUC/RUC on the upgrade list.