When you choose to rebuild a trip, you will be given the option to lock or unlock the stops that are already on the trip.  Locked positions are going to be things like the start and end points, the custom places you've added (like Grandma's house) or waypoints that aren't campgrounds (like the address to Mt Rushmore).  When you chose to rebuild the trip, you can lock the stops you don't want to lose in the "do over" and unlock the ones you'd like to "shake up". Then, when you chose new filters to find the new options, only those that are "unlocked" will change.