Originally Posted by
DCADeltaRoadWarrior
You can SDC when you have a PAID premium cabin seat to another flight, if there is availability.
That doesn't apply to RUC's or complimentary upgrades that have already cleared.
It doesn't apply to complimentary upgrades--where you must SDC based on availability of the original coach fare class and then either recapture RU/OU space on the new flight or go onto the (gate) upgrade list for the new flight--but it does apply to confirmed instrument supported upgrades into RP/OP inventory. However, if the ticket hasn't yet been reissued into first/business class, this might cause trouble when you call to SDC.