Besides losing the fare difference if the earlier flight happens to be cheaper than the original purchased one, SDC within 24 hours "if the same booking class is available" is also very hard to achieve since most discounted fare classes are no longer available 24 hours before departure. That is currently also Delta's policy and it rarely works as flights are typically fully booked close to departure time. It will be interested to see how the other two traditional carriers (DL & AA) will adjust their rebooking fees as Delta currently will issue credits for residuals if the new ticket is cheaper.
Also, how does this new rule affect changes to LATER flights as it relates to SDC and standby flying?