Log out of your facebook accounts.
Open big business account and post through Facebook.
As you are not logged into Facebook, you should be asked to sign into your account. Use the new Facebook account that you wish to change to.
Open the Facebook account you wish to change to.
Search for big business and open the app from here.
I can't remember which way worked, but I recently changed mine.
Hope it works for you guys too.
player name Amanda35, city name Squillion Square, from South UK,