If you have Switchvox version 5.x on an appliance with a FAX license, you will need to re-apply a new FAX license after you update to version 6.x. There is no purchase necessary, you just need to re-register for faxing under 6.0.
Once you have updated:
Open Tools > Digium Addon Products
Select Fax from the dropdown
Fill out the registration form
At that point, you'll be able to send and receive faxes again as you did under version 5.x.
Note: On version 6.0 and later only one Fax license is required and will handle multiple faxes.