The reservations rules have nothing to do with FastPass. That is solved by simply cancelling FP's booked outside 60 days when the reservations are changed. This has to do with making sure that people don't game the discount system. No different than changes on discounted air fares.
As to dining, if you are Club Level at one of the luxury resorts on the monorail, those concierges can find inventory not otherwise available and all have contacts at the specific venues whom they can call. You might not get the exact time you want, but if you want 8:00 PM and can live with 7:30 or 8:30 PM, you should be fine.