I haven't been that lucky with changing flights, although I have done it in the past. The main problem is that by the time I know which flight to change to, that flight is already sold out.
Hence my simply buying multiple tickets in advance and refunding the ones not used. Not ideal, but in my experience it is VERY hard to predict the time it takes to travel back to the airport.

Side note: ctrip is pretty nice about refunds. Normally they refund the predetermined portion of unused flights automatically, a few days after they realize I wasn't on it.