Specifically, an IPad to an IPhone?
On your iPad, go into the settings menu and log into your Facebook account.
Then set up the game on your phone and log into your Facebook. It will ask if you want to overwrite your current game (i.e. the one you just started from the beginning by loading a new install) with the existing account connected to that Facebook name.
Edit: Nvm. It works to get back the old account. Thanks.