Guests can start park hopping at 2:00 pm each day and can continue until closing time. That means anytime after 2:00 pm, guests are free to change parks.
Not all tickets let guests go to multiple theme parks in the same day. If that is something you want to do, make sure you purchase a ticket with the park hopper option.
Capacity limits on each park are still in place and you have to first check if there is space.