Selvarathnam, thanks for the suggestion [+5]. Yes, in fact with ELM beng introduced the 9.1 version will behave differently from the 8.x version. When you try to change the dns server you should get a warning like the one below
admin:set network dns primary X.X.X.X
WARNING: To avoid license synchronization failures, delete the
product instance from the Enterprise License Manager
managing this server's licenses before changing network
settings.You will have to re-add the product instance
after the network settings have been changed.
So just follow the steps as listed in the warning and it should be good.