Can I encrypt already existant unencrypted mail before I start using the mail-crypt plugin?

[mailinglist-subscriptions]

Hi,

I am using dovecot 2.3.16, along with postfix and a PostgreSQL database for managing virtual accounts.

I'd like to start using the mail-crypt plugin. However, I'm having a bit some difficulty understanding the documentation at 

https://doc.dovecot.org/configuration_manual/mail_crypt_plugin 

to reach my goal. I plan to ask questions about those issues by starting new threads in this mailing list. But before I even come to that, I'd like to investigate the following:

The above documentation only addresses a clean install and doesn't seem to mention encrypting already existent unencrypted mails, like my server has. Is it possible to encrypt those before I start using the mail-crypt plugin, such that it will be able to decrypt those messages as well?

If it is, I am assuming that how I would go about achieving that will be very dependent on the ultimate configuration I have in mind (pub/priv keys, etc.). So I don't expect a full-fledged guide. However, if you could perhaps give a general overview of what would be needed to achieve this, I would very much appreciate that.

Thank you.