It is not simply "fare type", there must be inventory in the originally purchased "fare class" for coach tickets. If you bought a V fare, there must be inventory in the V bucket for the flights you want to SDC to. Otherwise, you can only stand-by and the routing must be the same.
If you buy a C+(W) fare or F fare, there only needs to be an open seat (note that it must be a purchased fare, not an upgrade).
......or an upgrade supported with instruments that has been confirmed, although IME the ticket needs to have been reissued too.