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.
Updated: November 1, 2022
CRYPT7
Files with crypt7 file extension are typically found as older database backup files from some previous version of WhatsApp install messenger.
CRYPT8
A crypt8 file extension is related to the WhatsApp Messenger, an instant messaging client for Android, iOS, BlackBerry and Windows Phone. A crypt8 file stores encrypted database with chat history.