There is no direct way how you could ever convert db.crypt8 archives to crypt7 format and db.crypt8 to crypt7 conversion is unlikely.
However, the WhatsApp Tri-Crypt / Omni-Crypt program can be run as background process and convert old CRYPT8, CRYPT7 and CRYPT 5 archives to the original legacy CRYPT format, which then can be easily extracted with one of the available tools like WhatApp Xtract.