How about instead allowing SDC to happen at a 1 day window for GM/PM/DM? So if you are flying LGA-ATL at 7:00am on a Friday then on the Thursday before you can switch to any of the 15 other flights that day if seats are open. This is a much more customer friendly option and makes it easier to SDC on later flights without worrying about missing your original flight because of the current 3 hour time limit.
It would also be a wash for FOs...some higher medallions would SDC on your flight, but others would SDF off of it...