The only known way to convert latest WhatsApp archives (.crypt12) back to the original .crypt files is through app called Omni-Crypt, previously known as WhatsApp Tri-Crypt.
With this app, users can easily downgrade their WhatsApp backup files, however this procedure often requires Root Access to the device, which may not be convenient way for some users, because doing this often causes warranty problems. However, devices that run Android 4.0 or later do not require Root Access for Omni-Crypt to work.