When I did it (10 years ago), I went to the main ticketing/check-in counter. Once the agent checked her computer and saw that my request was doable, she took my ticket and issued me a new ticket for the earlier flight. At this point I was like any other passenger on that flight—my reservation was secure and I was no more likely to be bumped than any other passenger. And, if I recall correctly, this was about 4 hours before my new flight time. (I still had some time to kill.) The moral is: you don't necessarily have to wait till the last minute to find out if you can fly.

These days I think they won't allow you to be separated from your baggage. It's a security measure—they don't want you checking a bag containing a bomb and then skipping the flight yourself. I've heard that if they discover at the last minute that they're carrying a checked bag for a passenger that isn't on the plane, they'll unload the whole baggage compartment, if necessary, to find that one bag and remove it. It really pisses off the baggage handlers.