There is no direct way how you could ever convert CRYPT8 archives to CRYPT5 format. However, the WhatsApp Tri-Crypt program can be run as background process and convert all newer CRYPT8, CRYPT7 and CRYPT5 archives to the original legacy CRYPT format, which then can be easily extracted with one of the available tools like WhatApp Xtract.
Nonetheless, both crypt8 and crypt5 are more or less obsolete, so this conversion is likely as well.