There is an add-on to TB which allows import and export of emails in various formats to a separate storage medium (external disk, thumb drive, etc.). I suggest that you export your mail from the Win 7 PC and then import it to the Win 8 PC.

The instructions on how to download and install it can be found at http://barryduggan.info/exportMail.php

I downloaded and installed ImportExportTools and was able to export mail from the Win 7 PC but when I try to import it into existing files it doesn't work. The instructions you point to only give instructions on how to export files but not importing and the Thunderbird Import will only import files from other email clients and not from other files. How does one import the files from Win 7 Thunderbird into Win 8.1 Thunderbird?

You need to download ImportExportTools for your Win 8 Thunderbird. Then use it to import the files. See the discussion at https://freeshell.de/~kaosmos/mboximport-en.html for additional import format instructions and restrictions.