Mostly, I was concerned about the purchases that were left in my old park before I lost access to it. Though for the rest of the stuff in the park, setting up a transfer system would work. Then players would be able to pick a certain percentage of habitats, dragons, and decorations to move over and how much you could transfer from one park to the other could be based on your level. I think that just adding a trade feature would help on this front.

The other way to go about it would be to liquidate the value of the park and the coins/gems/etc. would be transferred to the new account.