I'm not a historian, but unfortunately it looks like this fell through the cracks.

At the end of the Year of the Five Emperors, Byzantium refused to surrender to Septimius Severus, who destroyed the city and then rebuilt it in 196 CE. Constantine I founded "Nova Roma" on the site of Byzantium in 330 AD as a capital of the empire. After his death, it came to be known as Constantinople. Since the change to the city was so sudden, and the old Byzantium was dwarfed by the new metropolis, I speculate that the new name came quickly, but I will leave that to the historians.

Once the Turks conquered Constantinople, the city slowly started to be colloquially referred to as Istanbul, deriving from a Greek phrase that meant "to-the-city". It was not until the collapse of the Ottoman Empire and the founding of the Republic of Turkey, and the subsequent relocation of the capital to Ankara, that the name of the city was officially changed to Istanbul in 1930. It had been known within the city and in Anatolia as Istanbul far earlier than 1930, however.