Yes, you can take a look around without losing anything. When you select another park it will give you a warning saying:
"Made Plans at Another Park
Guests can only make FastPass+ selections in advance at 1 park per day. If you keep these Guests in the party, their other FastPass+ selections will be canceled and replaced. You’ll get to review and confirm this change before it’s final. "
Just select "Continue with this Guest" for all people in your party. Nothing changes to your original FP unless you select a new one in the new park (and even then it will make you confirm the change).