There are several different ways to initially encrypt or re-encrypt your existing data. The method you choose depends on the security needs of your database. Methods include:
Allowing the normal course of database updates to encrypt the data. If a new encryption policy is applied, every time an block is updated, the block is encrypted with the new policy prior to being written back to the database. If the data you are encrypting is updated regularly, all the records are encrypted (for the first time, or with the new policy) during the normal course of operations.