The change in identity took place during the Greek National Revival (also known as the Greek Renaissance) in the early 19th century and specifically during the Greek War of Independence (1821-29). This is when Greeks dropped the Roman identity and 'turned back' to embracing their ancient Hellenic heritage as a new identity.

The idea of nationalism and the nation-state was spread by the French all across Europe during the Revolutionary/Napoleonic Wars and Greeks were influenced by this idea, as were the Italians and Hungarians and Germans and many other peoples who lacked a unified single nation-state. The first of such Hellenic nationalist revolutionary groups was Filiki Eteria, founded in 1814 in Russia by Greek exiles.

The Byzantine Empire was never a Greek nation-state and in fact as the Eastern Roman Empire it continued to be a multi-ethnic and multi-lingual political entity just like the Roman Empire of antiquity, albeit the dominant language in the East was Greek. The Greek nationalists wanted a nation-state, not a re-establishment of the Byzantine Empire.

Now, the reason the Roman identity was swiftly abandoned was primarily because of its association with the Ottomans, who were despised by the Greeks. After the fall of Constantinople in 1453, the Ottomans had claimed they were the continuation of the Byzantine Empire and for example Mehmed the Conqueror took the title Kaysar-i Rûm, meaning 'Emperor of Rome'. Earlier the Seljuk-Turks had called their state Saljuqiyān-e Rum, the Sultanate of Rûm. So there was this dreaded Roman association with the Ottomans, and the Greek revolutionaries wanted a new nation-state devoid of any kind of historical connection to the Ottoman Empire. Greek nationalist writer Adamantios Korais, a key figure in the war of independence, was in particular critical of how subservient the Byzantine/Roman elite and clergy had been towards the Ottomans in the past.

After independence the Greek-speakers in the independent Hellenic nation-state identified as 'Greek', however the millions more Greek-speakers who lived in the Ottoman Empire continued to identify as 'Roman' and did not adopt the new identity. When Greece united with some of the former-Ottoman Aegean islands (Limnos, Lesbos, Chios) in 1912 after the conclusion of the First Balkan War, the Greek-speakers there took some time to adjust to the change of identity from Roman to Greek. When asked who they were by the Greek soldiers who liberated the islands, they answered back Romaíos.

This was because the Ottoman Empire did not recognize secular ethnic-groups, but rather only religious groups - Eastern Christian (Roman), Armenian-Christian, Muslim, Jew, etc. So under 400 years of Ottoman-rule, the people could not identify with a secular identity and though the Greek nationalists/revolutionaries tended to be very religious people supported by the Church, they saw the secular Hellenic identity as a mark of defiance against the Ottomans and a rejection of their society.

Philhellenism should also be mentioned as a reason why Hellenism was promoted in particular, and this was a diplomatic strategy by the Greek nationalists to get Western aid and sympathy. Ancient Greece was a very popular study in democratic Western countries, and seen in a very positive light with the creation of democracy and philosophy. By embracing a Hellenic identity, the nationalists hoped the Western powers would support their cause out of sympathy and this worked. By 1827 the British, French and Russians began supporting the Greek revolutionaries and public support for the Greek cause was very popular throughout Europe in the 1820s. Without Western intervention, it is probable that the Greek revolutionaries would have lost the war.