Were you trying to SDC (Same Day Confirm) or were you trying to stand by? You mentioned SDC but then quoted the rules for standby.

In any event, airlines do not like to make flight changes requested by the passenger after bags have already been checked for a particular flight. Of course, the airlines can jerk you around with flight changes at their convenience and they can also put your bags on a different flight at their convenience (as you found out). I think this stems from the days before checked-baggage screening when someone could check a 'tainted' bag and then beg off of that flight so when it blew up they would not be on it.