You cannot simply convert older Whatsapp message backups (for example .crypt8) to a later variant (.crypt12), nor you can import old files to newer version of Whatsapp.
However, you can trick Whatapp so it restores the backup by putting the .crypt8 in the default folder on external storage, uninstalling WhatsApp and reinstalling it. This will prompt old backup import wizard and convert the backups to the latest available version, essentially achieving crypt8 to crypt12 conversion, or to any latest version that might appear in the future.
