Thanks Dan - It's a reasonable guess!

Following link has some explanation about why it has not worked on Diego - (The documentation of cf restart-app-instance should have this mentioned.)
https://github.com/cloudfoundry/cli/issues/777 

As per above link - Diego team has done some work to propogate env var changes during restart-app-instance. Does anyone here know more about it?
Is there any way to get env vars reflected using cf restart-app-instance or any other means without downtime?

On other hand I also thought about using copy-source but that command require the target app to exist. Can we clone the app?

Appreciate any help on this - Many Thanks !
~Preetam