Backward compatible changes for namespace change

Namespace change is backward incompatible. Can we really change Namespace? If yes, what is the process to make backward compatible changes for the namespace change ?

There is no way to make a backward-compatible change to a namespace. See RFC 7950 - The YANG 1.1 Data Modeling Language
You would need to make a non-backward-compatible change, make the necessary changes to your application(s), and have users of the northbound interfaces make changes too.