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.
Convert WhatsApp Messenger encrypted message database backup to WhatsApp Messenger message database.
We found 2 software records in the database related to .crypt9 to .crypt8 file format conversion.
Google Android software - convert crypt9 to crypt8 on Android
Popular instant messaging client for Google Android
Convert from crypt9 file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned crypt9 to crypt8. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Android app used to decrypt and convert CRYPT files