After you install a new EP, move your downloads folder to your desktop or my documents (or somewhere out of the game, don't delete it, just move it), open body shop from your new EP and body shop will automatically create a new downloads folder in your game.  Then, move all of your downloads out of your old folder and put them your new one and your stuff should show up in your game.  All you have to do after that is make sure what ever you transfer is compatible with your new EP.  If something isn't, check the site that you got the download from for updates.