My provider (mailbox.org) offers PGP encryption at rest. Which means incoming mails get automatically encrypted with my key. This only works for new, incoming mails though. When I migrated, all my old mails got transferred unencrypted.

Is there a way in Thunderbird to encrypt my old, unencrypted mails? The only other way I can think of is to forward all old mails to myself, but I would prefer to not do that.