The neighborhoods are in the worlds folders of the game. I have found you can create a neighborhood by deleting all the content and remaking either sunny view or the other neighborhood. If you save a duplicate file under another name you can in fact make another world but you still can customize the lot sizes or upload it at the exchange.
PS changing a saved file into a world file doesn't work it gave me a error when I tried but it did show in the menu.
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PS changing a saved file into a world file doesn't work it gave me a error when I tried but it did show in the menu.