You could try using a Thunderrbird Portable instance in windows …
- Config. your GSuite (Old Gapps) account(s) in thunderbird as POP and download your mails.
- Config. your BOX accounts in Thunderbird as IMAP
- Copy your earlier downloaded emails from your Gapps account(s) to the equivalent account/ folder(s) in BOX ones… All emails will be uploaded to your BOX ones.