You can convert .crypt12 WhatsApp backup files to the original .crypt file using WhatsApp Tri-Crypt / Omni-Crypt app, but you cannot convert it to any of the previously used version, including crypt5. Once you have the .crypt file, you can extract the messages with other tools. Said that, crypt12 to crypt5 conversion is something that was perhaps done years ago and nowadays is most or less obsolete.
Open CRYPT12 file Open CRYPT5 file