If you bought it through Disney I don't think you can change it to a one-park-a-day ticket, unfortunately.
You can go into your park reservations and change your starting park as long as the park you want to start in is available. If not, hopping starts at 11:00, so you could either sleep in and go to your preferred park at 11:00, or you could go into your current reserved park and look around for a few hours. Hoppers are super handy, you might like it.