Paul wrote:
[...]

I don't know a thing about this, whether it'll work or not. I
would suggest backing up all materials before attempting anything fancy.

https://addons.thunderbird.net/en-US...xporttools-ng/

FWIW, I've used (an earlier version of) this software to import my
(exported) email from Windows Mail into Thunderbird. Worked fine.

By importing the exported mail from the new profile into the
(recovered) old profile, the new mail should be merged with the old
mail.

An even safer method would be to export mail from both the old and
new profiles and then import them into a third - even newer - profile.

That way, if you keep backup of the old and 'new' profiles, you can
always go back in case anything goes wrong.

[...]