'Change party' is intended to add people to your existing FP selections whereas 'modify' is best to change the time or selection of existing FP for one or more guests. I have always used the 'modify' option. Don't worry about messing things up. You can go all the way through selecting people, attraction, and times up to the final confirmation page without losing your existing FP. IF you don't complete the final confirmation of new FP, the original ones will still be there!