So reading the responses to Scott's announcement, I have some comments and pipe dream suggestions:
1) With the 12 hour window and the 1/2 price for gold/plat, I certainly feel better about it... not 100%, but better.
2) How about, if the new flight pushes back prior to the original departure time, you charge the fee. If it pushes back at or after the original departure time (e.g. you switch from the 8:10 to the 6:05 but end up pushing back at 8:15), no fee is charged.
3) Your price structure and window offers a unique opportunity to make the fee time sensitive: How about a $4 ($2 for plat/gold) charge for every hour or part thereof for time change. Flying 12 hours later: $48; Flying 2 hours early: $4.