Although you cannot simply turn strong encryption on or off in an existing database, you can choose from three options when it comes to implementing strong encryption. You can either create a database from scratch with strong encryption, you can rebuild an existing database and change the encryption status at that time, or you can use the CREATE ENCRYPTED FILE statement on an existing database.
You can rebuild the database to unload all of the data and schema of an existing database. This creates a new database (at which point you can change a variety of settings including strong encryption status), and reloads the data into the new database. You need to know the key to unload a strongly encrypted database.