Since there are separate buckets for on site vs off site, I think there are two possibilities of what might happen.
1) Disney will cancel all of your reservations and you will need to remake them in the day guest bucket (subject to availability)
2) Disney move your current reservations to the new bucket, if there is availability. They would cancel any reservations for a park that has no availability in the day guest bucket. For example, if you had a reservation for DHS with the on site stay, but day guests that is full to capacity it would cancel that one. But if you had one for EPCOT and there was availability they would move it for you.
In all honesty, I would expect number one. It would be the same as FP+ and losing it if you cancel your resort stay. You would then need to remake them