If a Turkish Cypriot is a Christian, does that make them a Greek Cypriot?

Under the millet system, which is still recent memory in former Ottoman countries, creed was the determinant of identity. If you were Orthodox you were Rum/Romios, if you were Muslim you were a Turk—no matter what your ethnicity, and what your main language was.

So a Greek Cypriot that converted to Islam 200 years ago was deemed a Turkish Cypriot. The penalties on apostasy from Islam were in full force, but yes, if a Turkish Cypriot converted to Orthodoxy, he would be deemed a Greek Cypriot. And many Turkish Cypriots would have spoken Greek anyway.

That kind of thinking was done away with in the 19th century through nationalism; and there wasn’t a lot of precedent for conversion to Orthodoxy anyway (though the Orthodox church does commemorate the few such precedents as martyrs). So now, the answer is no.