We have found one software suitable in our database for this conversion.
The WhatsApp Tri-Crypt Android app supposedly is able to convert between various CRYPT, CRYPT5, CRYPT6, CRYPT7 and CRYPT8 files so it is your best bet if you need this conversion. However, it does seem unlikely, given the nature of these files.
At best you first decrypt CRYPT7 to the original CRYPT and then again crypt it to CRYPT8, but the purpose of this would be a little mystery.
An application for Google Android operating system, which is used to monitor and convert WhatsApp .crypt5, .crypt6, .crypt7 and .crypt8, .crypt9, .crypt10, .crypt11, .crypt12 backup files to .crypt legacy encryption and to .crypt5, .crypt6, .crypt7 and .crypt8, .crypt9, .crypt10, .crypt11, .crypt12 decryption / encryption. It is available in free and pro paid version.