Well, the city of Constantinople was attacked by Mehmed. And it has been called Istanbul since the 10th century. I got this statement for the wiki page of the Names of İstanbul:

[i]"After the creation of the Republic of Turkey in 1923, the various alternative names besides İstanbul became obsolete in the Turkish language. With the Turkish Postal Service Law of March 28, 1930, the Turkish authorities officially requested foreigners to cease referring to the city with their traditional non-Turkish names (such as Constantinople, Tsarigrad, etc.) and to adopt Istanbul as the sole name also in their own languages.[15] Letters or packages sent to "Constantinople" instead of "Istanbul" were no longer delivered by Turkey's PTT, which contributed to the eventual worldwide adoption of the new name."[/i]