The use case for downgrading from an Act! Premium database to Pro is not uncommon. Users needs change, and they may no longer have need for the advanced security, web publishing, API, or networking features included with Act! Premium, and they wish to switch their team to Act! Pro. While this decision should not be made hastily, it is technically supported to downgrade a database from Premium to Pro. Please note this should not be done for workgroups larger than ten users. The following is sourced from a KB article at http://help.act.com.
Here are the steps to downgrade your database from Act! Premium to Act! Pro:
Note: Reopening the database in Act! Premium will convert the database back to the Act! Premium format and you will not be able to open it in the Act! Pro version unless you downgrade it again.