As far as we know, there is currently no way how to convert or decrypt .crypt9 files to something else. This is usually done by users that try to access message history of latest versions of WhatsApp in attempt to read it.
Considering .crypt9 files were only used for very short time and moved to higher version soon after, this crypt9 to crypt8 conversion is most likely very rare and more or less obsolete.
Unfortunately, there is no detailed software record in the database that match this conversion.
CRYPT9
Files with crypt9 file extension can be found as encrypted chat history databases from latest version of WhatsApp Messenger instant messaging client for Android, iOS, BlackBerry and Windows Phone
CRYPT8
Files with crypt8 extension are old encrypted backup files from WhatsApp Messenger instant messaging client for Android, iOS, BlackBerry and Windows Phone.