Adding waterparks to tickets after a few days?

[Belle5]

Should we decide to add waterparks to our 9 day base tickets after a few days of using them, that is not a problem, is it? The cost would be the same as if we had added the parks from the first, right? We are undecided as to whether we want to do the water parks this time, and thought it might be smarter to wait and see if we are in the mood once we've been there a while.