You probably already have some records in your database. Encryption key must be chosen before first enrollment and then kept constant. If you change encryption key on a non-empty database, database error is generated to prevent data corruption.
In order to migrate old data to use new encryption key, you can use IDKit PRO SDK to load all records from the old database and register them in the new database with different encryption key. IDKit PRO SDK supports multiple simultaneous connections (either AFIS connections or direct ODBC connections), so it is quite easy to migrate data between two databases.