Your Fitbit account can only ever be connected to one Pacer account at a time. If you have already set up your Fitbit to connect to your Pacer account, you will not be able to connect your Fitbit to a different Pacer account until you deauthorize the first Pacer account.
Deauthorizing your Pacer can be done from the Fitbit website:
1. Go to the Fitbit website (www.fitbit.com)
2. Log in using your Fitbit credentials
3. Go to your Fitbit account settings
4. From account settings, go to "Applications"
5. Look for Pacer and select "Revoke Access"
6. Pacer and Fitbit are now disconnected. You are now free to re-connect your Fitbit to a different Pacer account.