Originally Posted by
sethb
My experience is on Delta. When I upgrade via an instrument, the new ticket is definitely First Class (or Business). The instrument isn't "the same as cash" but the resulting ticket has the same rights (e.g. SDC to any other flight with an available seat).
Definitely not the same on United. SDC from an upgraded seat will drop you back into Y. Presuming there is upgrade space available you can then request to be upgraded, but depending on the agent involved it may or may not be a trivial exercise.