When and why was the name of Constantinople changed Istanbul?

Answer 1

The short answer is that in the modern era, after 1923, the new country of Turkey wanted to use only the name Istanbul. More information from other contributors is here:

other responses:

The name Istanbul is derived from the Greek word istimbolin which means in/to the city and was used to indicate Constantinople, which was often just called The City. It was already in use by the Turks in common speech before they conquered Constantinople in 1453. However, during the period of the Ottoman Empire two names seemed to be in usage until the 19th century. The name Kostantiniyye appeared as the place of provenance of documents of the court and the chancellery and on coins up to the 17th century and reappeared on coins in the 19th century. The name Istanbul was associated with the highest magistrate (Istanbul efendis) and the highest military commander (Istanbul agasi).

With the establishment of the Turkish Republic in 1923 Istanbul became the sole name of the city. With the Turkish Postal Service Law of 1930 Istanbul became the official name and foreigners were requested to adopt this name in their languages and drop other names. In Europe the use of the name Constantinople had remained widespread. The Turkish postal services would no longer deliver mail sent to Constantinople instead of Istanbul.