if you have an external hard drive then just copy the itunes folder to it. then when in leopard open itunes while holding option and choose the folder.
if you dont have an external then in itunes choose file->back to disc. this will backup everything it itunes like music, playlists, videos etc to a dvd. if its larger than one dvd then it will ask you to insert another. when in leopard just insert the first dvd and your ready to go.