Explanation/Reference:
The keystore is tied to the storage processors, so care must be taken when performing service operations in order to preserve the encryption keys. In the event that the chassis and both storage processors need to be replaced, both storage processors should not be replaced at the same time. Instead, the chassis replacement should be followed while retaining one SP until the array is back online before replacing the second SP. In the case that storage processors have already been replaced at the same time and the keystore was lost, the keystore may be restored from an external backup.
References: http://www.emc.com/collateral/white-papers/h15090-emc-unity-data-at-rest-encryption.pdf (page 10)