Along a long enough timeline, you can backtrack and find a different ethno-religious identity for everyone. In the context of your post, what comes to mind-to me- are instances where one groups identity was a close run thing. Or, where the identity has changed to a point that the old identity is starkly contrasted to the present. Some things that come to mind are...
Western North Africa, as well as Egypt, was one of the heartlands of Christian thought and development. So what's your ethnic group there....Berbers? Punic types? To be honest I still can't look at "Arab" Egypt's identity and NOT think of the Copts....they're still very much around.
Spain could have remained an Islamic country semi-permanently had things gone differently. Not sure of the numbers on how much of the population was Muslim, though.
The inhabitants of Anatolia, pre-Turks, were very Christian. Again, it was a center of the faith- Paul, Nicaea, Constantinople, etc. Today, the genetic basis of those individuals is still of virtually the same stock as it was then, but they have been Turkicized culturally and the Christian element is nill. When does anybody ever think of a Turk as a Christian? (though personally I've met some).
Maybe the state of Massachusetts? It's changed BOTH it's ethnic and religious identity. Used to be an Anglo, Puritan, reactionary breeding ground. Not really a friendly place for Catholics. Now, thanks to the Irish and Italians, it's THE Catholic state. And with California, probably THE liberal state, too.
How about the identity of the Europeans as a whole? Its brand of Christianity got exported to the world and is looked back on as the heartland of Christianity (even if this is wrong). Now, the majority of it is pretty heavily secular, and it's the place Benedict bitches about whenever he wants to criticize "materialism and relativism".
The staunchly Catholic nature of Latin America is changing rapidly as we speak. Personally, I find this the most interesting of all.