Changing a FP vs Adding a FP

My teenage daughter and I intend to visit EP on a level 2 predicted crowd day in the AM. We are going to ride Soarin, TT, & MS Green only and then leave for MK (crowd level predicted at 5).

I plan to get advanced FP for TT and MS Green (will have breakfast at GG and then head to Soarin w/o FP). We have no need for the third FP in EP. Is it easier to book any third FP for EP and change it to a MK attraction (SM) once we tap in to the first FP or is it better to leave the third open and add it later? Does it make a difference which method we use?

Better to not prebook the 3rd at all. then day of, once you use/scan your first prebooked FP, you can book a 3rd for MK.