Now, the new gameplay mode is linear, and you can't visit the parks already completed anymore once you accept to travel to the next one.

For the time being, your old park will be closed once you reach the milestones needed to travel to the new theme park. Now, you don't need to max up all your rides, but only complete the minimum requirements to switch to the new waterpark. The team is studying to keep the old parks open in further updates so players can complete all the rides and levels. Thanks for your patience!