I've never had someone not allow this, and I've tried about 5 times. I've had 2 times where they confirm my ticket, rather than standby, so there's certainly some variability. The only screw up was one flight when I checked my bags, and very explicitly told the agent to put them on the flight I was going to try to take earlier, and she sent them on the original flight. Of course that was the one time I didn't check the tags....