I used to do this all the time too -- sometimes I would even travel on the following day -- and I was never charged a change fee, until about three years ago when the airlines became much less lenient. On most major carriers, even with their cheapest published fares, you can put the value of the ticket (minus change fee) towards future travel within a year of the original travel date. Delta was the only exception; their requirement was that you book your NEW travel by the original travel date. But Delta reversed that policy about a year ago. As far as I know, you can still travel standby but you have to pay a change fee AND the difference in fare. I don’t think there is anything unethical about this at all. But you are taking a big risk in that there’s always the chance that there won’t be seats on the flight you want.