@Praveen_More the process is documented in The Create stage FAQ   and discussion Case ID generation mechanism

A forum  user gave a good overview in PyID  Also see How to get the ID of Current Case?

Another user explains how modifying this is not a good idea: 

Store pyID with a different value than the one generated by Pega

Modify pyID for existing resolved cases

@KenshoTsuchihashi would you be able to provide some assistance to this user?

@Praveen_More

For achieving this business need, until create stage process is getting completed you should not persist the case.

You could enable the Temporary Case check box, which will be avail in your pyStartCase flow under Process Tab.

This configuration will allow system not to generate case id until the case is getting persist.

After the create stage completed you can place OOTB Persist Case smart shape in your primary flow as first step, then system will generate only case id after system reached the Persist Case smart shape.

You can extend the OOTB smart shape activity to generate case-id based on the business need.

Modifying the case-id after it got generated would not be a recommended approach, because pyID and pzInsKey will be tightly coupled and by using this 2 values Pega will establish link between work and many back-end table also.