I’m impressed by the size. :) Out of curiosity, you must be having separate folders to store them in, right? And you would like to upgrade from Thunderbird version 17 to the current version (45.4 at this point)?

I am not aware of any incompatibilities regarding mail formats or changes to other important files that would cause issues after a Thunderbird update - I have a (POP) account containing the same local files ever since (checking...) Thunderbird 2.0.0.5pre on one system, but that could even have been version 0.9 if it wasn't for an intentional fresh mailbox due to a system upgrade around some Christmas / New Year. ;)

If you are using POP (I think you do) and have an additional 40G of free local diskspace, you could however create a backup of your entire profile (including the parent file called profiles.ini) first, so if anything goes wrong, you could just restore things if needed.

If you use IMAP (but I doubt if your ESP provides that capacity), I think you could even do the same.

Note that if you have any Eudora mailboxes you would like to import in the near future, it may be best to do so now, since current Thunderbird versions no longer can. Version 31.8.0 is the last one to support that feature (at least for Eudora), and some people have had better results using version 17.0.9(esr) after they used version 45.x and downgraded to version 31.8.0 for that purpose.

Thank you for your response. You answered my question. Yes I have a about 100 folders sorting out the files across 6 accounts. I started with Netscape and followed its transitions through the years when it became Mozilla. I still use only Mozilla FF and TB. I install them on every computer of friends and family. I moved to TB at about 2.0 and for some compatibility reasons I cant remember, stopped upgrading at V17.

Again, Thank you MikeH