Not necessarily. If you don't have a reservation, you just need to wait until 2 PM to enter any of the parks. If you do have a reservation, you just need to scan into the park you have a reservation for before you can travel to another.
For example, if I have an EPCOT reservation on one of the AP preview days but I want to go to Magic Kingdom - I can scan into EPCOT, immediately leave the park, and then travel to MK and scan in there. The rule used to be you couldn't park hop until 2 PM regardless of reservations, but now the "after 2 PM" rule only applies to no reservations.