Using some specialized programs it is possible to transfer and restore older Whatsapp backup (*.db) and then use the chat backup function to generate a new backup, which uses CRYPT8 format. You could get a CRYPT7 with this method as well, but latest version of Whatsapp uses CRYPT8.
Convert WhatsApp Messenger message database to WhatsApp Messenger encrypted message database backup.
We found 2 software records in the database related to .db to .crypt7 file format conversion.
Blackberry mobile phones and devices software - convert db to crypt7 on Blackberry
Cross-platform smartphone messenger also available for BlackBerry
Convert from db 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 db to crypt7. 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.
Windows Phone / Windows 10 Mobile software - convert db to crypt7 on Windows Phone
Instant messaging client for Windows Phone OS