What happens if I link my current Account with a Facebook Account with old game data?

If you attempt to link your current Guest Account to an old Facebook Account that was used, you will be prompted to choose whether or not to continue and load your old data.

If you do not wish to overwrite your current game data with your previous data, please choose Cancel at the prompt.

Note that if you go ahead and overwrite your current game data, there is no way to restore it.