You cannot simply convert older Whatsapp message backups (for example .crypt8) to a later variant (.crypt12) nor you can import it to already Whatsapp. But it might be possible to trick Whatapp, so it restores the backup by putting the .crypt8 in the default folder on external storage, uninstalling WhatsApp and reinstalling it. This should actually prompt old backup import wizard and convert the backups to the latest available version, which may even be higher than .crypt12.
With that said, .crypt8 files are pretty old so nowadays crypt8 to crypt12 conversion is something very rare most likely.Open CRYPT8 file Open CRYPT12 file
Android app used to decrypt and convert CRYPT files