first backup their profile. assuming it is stored in the default location. (check the account directories for each account to make sure) open the windows file explorer and type %appdata% into the location bar. I discuss it more here https://thunderbirdtweaks.blogspot.com/2018/02/copy-thunderbirds-profile-to-new.html copy the Thunderbird folder and sub folders as a backup. On the same disk works, as you are changing nothing but making a fail safe. Thunderbird V6 was 32 bit, if possible download a 64bit version as it will install in the program files directory and the V6 will remain, unchanged in the program files (x86) directory. SO you are not messing with the program and you have a backup of the data to just replace if it goes pear shaped. You can also download any released version of Thunderbird from here if the need ever arises http://ftp.mozilla.org/pub/thunderbird/releases/ now the install should "just work", but leave time for it to not be smooth. There can be issues with the profile actually getting lost in the update. That issue I discuss here https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html But I am not aware of an instance where full data recovery in the same profile, or a new one by copying data, was not possible from an upgrade. Even my own disastrous one to V78.

Thanks Matt for this info. I'll give it a try and see what happens. The biggest thing is BACKUP!!! Always a good first thing to do when doing any changes! Again, Thanks!