Let’s say you want to rename a large metropolitan city with an ancient and long-established name. By way of example, let’s say you want to rename the historic city of “Constantinople” in Asia Minor (modern Turkey) to “Istanbul”. How would you accomplish this? How would you get people to accept the name change and start using the new name?

You pass a law authorizing the post office to “return to sender” all letters, packages, and other correspondence addressed to the old city name, that’s how. According to this Wikipedia entry for Names of Istanbul:

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. Letters or packages sent to “Constantinople” instead of “Istanbul” were no longer delivered by Turkey’s Postal Service, which contributed to the eventual worldwide adoption of the new name.

Bonus Question: What about the previous name change from Byzantium to Constantinople? How did the ancient Romans get people to accept and use their new name for this old city?