Over in the enigmail mailing list:

https://admin.hostpoint.ch/pipermail/enigmail-users_enigmail.net/2017-February/004323.html

Patrick Brunschwig writes about converting an encrypted message into a cleartext
message:

If the message was signed and encrypted in one step, then the signature
will be removed as well. GnuPG does not have a functionality to extract
the signature from the encryption (which would theoretically be
possible).

If this functionality was provided, then enigmail could presumably preserve
signatures on the transformed message.

I don't personally think this is a high priority, since i think a better
approach (over storing the message as cleartext) is to simply cache the session
key to facilitate future decryption without secret key access.

But i wanted to record the observation of the missing feature here.