If you know how to change the terrain brushes on preexisting parks, then it is a similar process with a different button.
Go to "my parks" and there should be a button that says "edit as new scenario". Next to that button is a small square one that, when clicked, will give you option to change the biome of the park for the new scenario.
Although similar, the button next to "resume" does not allow for the biome to be changed, just the brushes.
I have had a problem using this feature, though. When changing the biome to anything other than the original biome, the terrain around the park gets rised up really high and is really hard to make even with the surrounding "skirt" of the park.
I made a bug report about it, so if y'all experience the same thing, please let me know!