Yes, but there is a well-recorded instance of population replacement in that case.
Greece is one of the most consistently-populated areas in the last several thousand years. I would feel comfortable assuming that more populations have left the area that is modern Greece than have entered it.
I admit I’m not as knowledgeable in modern eastern Europe as I am in Classical studies, but if my peripheral knowledge relevant to Biblical studies and Medieval Europe count for anything, there isn’t much there to claim anything close to an American-level of emigration to Greece after the Roman empire.
According to this site the first recorded positive migration (that is, more moving in than leaving) to Greece was in the mid 1970s. I had a teacher who was a Greek immigrant, who recalled that Greeks were not considered “white” when he moved to America as a child in the late 1950s.
I think there’s a good argument to be made for a population fairly consistent with that of Classical Greece.