None other than what others have suggested. If your currently booked flight isn't showing late, then there is a 99.9% they will charge you the difference in fare to board the earlier flight.

There's on obvious reason for this: If they let everyone do what your trying to do without penalty. Then quite a few people would just book the cheapest flight for that day and either show up early or late, etc, and ask to get on another flight. Although, there is a flat tire rule that I'm sure gets abused...