It is only possible to decrypt data if it has been encrypted at an earlier stage. When decryption is made all personal data on closed tickets will be restored and encryption when tickets are closed will stop.
If you are insecure if you have set encryption or not, open you end-user account and go to User button > View profile > Encryption key. If encryption has been set there will be no opportunity to generate new key, only information on when the latest key was generated.
As end-user you decrypt here: User button > Personal data protection > General > Decrypt data. Enter the saved key and click Send. Decryption is done immediately without the GDPR-handler having to approve it.