When I started playing Karaman a short while ago, I noticed that in AGCEEP, Smyrna starts turkish / orthodox, just as chegitz guevara suggested. However, the city of Smyrna was mainly Greek well into the 20th century. Only the mass deportation in the 1920s, where all Turks had to leave Greece and all Greeks had to leave Turkey, changed this. Smyrna was even called "Gavur İzmir", unbelieving Izmir, because of its Christian majority. Of course, the province includes more than just the city, and the last census of 1914 shows a large turkish majority for the province as a whole. However, this was five hundred years later; it is reasonable to assume that by 1419, the province, and especially its economic elite, was Greek dominated, and that the province culture changed sometime later.
The culture of Anatolia changed gradually over centuries, starting well in the Middle Ages. So by 1419, in much of Anatolia, Turkish was the main language and culture; the region around Smyrna, however, came under Turkish rule only in the 14th century, not counting a brief interlude from 1076-1104. Thus, I think it is fine to revert to the "vanilla setting" for Smyrna with Greek/Orthodox, but to change this accordingly either with "City of Man's desire" or through another event.
I confess, however, that I am deeply unsatisfied with the culture changes in events like "City of Man's desire", and that the Byzantium fantasy events with their chain of decisions necessary to change culture are much closer to what happened in Anatolia (although the direction was the other way round, of course).