First of all it is Konstantinoupolis, not Istanbul which incidentally comes from the Greek words, "eis tin polin" which was pronounced, "eis tin bolin" and was later corrupted to Istanbul. Going back even further, it was Byzantium, still Greek.
Iran forever, you are right, to you cavemen it is still Constantinople. You are about a millennium behind from the rest of the world. When you catch up you can then start calling it Istanbul.
Then again the fact that you will always be living in the stone ages means it will always be Constantinople to you.
