Is possible to Encrypt a form after it's been made un-encrypted?

Is is possible to Encrypt a form after it's been made un-encrypted?

Thanks

If I understood your question correctly, you asked that if the form is encrypted the previous submission data will be encrypted as well as with the new ones. If so, only the submission data which are received after the encryption process will be encrypted and the previous data will be left as the unencrypted.

Could you please check this guide in order to have a better idea about encrypted forms: 

Encrypted Forms and How to Use Them

Let us know if you need any further assistance.