Here's how I'd try it:
WARNING: I'm not sure if an older version of mail can re-import newer mail files.
It MIGHT be necessary to "export your entire mail folder" to mbox format before proceeding.
To do this, I think you'll need to export EACH AND EVERY mailbox separately.
There may be easier ways, I'm not sure, others will have to jump in with their experiences.
I believe there ARE utilities out there (not free) that can assist in this operation.
AGAIN, I'm not sure if you can do the following to move mail from Catalina to High Sierra.
Mail.app (in High Sierra) may refuse to import mail that has been "touched" by Catalina.
Did you keep an older backup around somewhere that represents your mail back BEFORE you upgraded to Catalina?
In any case, this is what I'd try:
First, you'll need a drive large enough to hold the "mail" folder (keep reading).
Format that drive to HFS+ (Mac OS extended with journaling enabled, GUID partition format).
Next...
Connect the "mail backup" drive to your Mac
Navigate to home/library (so you can see it in the finder).
Locate the folder home/library/mail
Copy the ENTIRE FOLDER to the backup drive.
Disconnect and set this aside in a safe place for now.
When you have "the downgraded OS" up and running, do this:
Connect the mail backup drive
Open Mail
Go to "import mailboxes" in the file menu
I'd try "import data from Apple Mail" (which is the default setting)
Then, I'd navigate to the external drive and select the "Mail" folder as the "import target".
See if Mail will "bring it all in".
You'll probably need to do some re-arranging, etc., once it's all imported.
This is how I'd try it.
I can't guarantee success, but if I recall, I've "re-imported" old mail back into Mail before using this method.
AGAIN, this MAY NOT WORK trying to import "newer mail" back to an older version of the OS.
EDIT: (IMPORTANT)
I keep a copy of Catalina on an external drive. I have never used it as my "boot OS" (I'm using Mojave).
I just mounted the Catalina drive in the finder to experiment with it.
I opened Mail (using Mojave), and chose to use the "Import Mailbox" option I described above.
Mail was able to import some mailboxes from Catalina (there's essentially only mailboxes there, with no internal mails, as I said, I don't use it).
I'm going to GUESS that if it will import "just the mailboxes", it will probably import mail "inside" as well (but again, that's ONLY "a guess").
So... it MIGHT be possible to import mailboxes from Catalina to Mojave.
Again, I CANNOT PROMISE this will work for you.
Note: You will find your account on the "data" partition in Catalina.
Also... the "library" folder is invisible by default. You will need to use some kind of trick to make it visible before Mail will be able to "see that it's there" and access it.
