Crete and Greek Macedonia in 1923, by the Population exchange between Greece and Turkey. Done by expulsions of the kind currently frowned upon (Ethnic cleansing), but which happened quite a bit after both WWI and WWII. On the Greek side, at least, the process appears to have been relatively orderly. Well, as orderly as that kind of thing can be…
EDIT: I forgot: yes, 1923 was orderly; the 1890s in Crete were not, and sectarian violence forced many of the Muslim population to leave by 1900: Cretan Turks. The Muslim population had already fallen from half the island to a third from 1800.
Crete has the distinction of being de-islamised twice. The previous occasion was the Emirate of Crete in the 9th century, which had a reconquista by the Byzantines in 961. Byzantine chroniclers boasted that the entire population was massacred and repopulated from Cappadocia; but of course Nicephoras Phocas was not Reinhard Heydrich, and it’s unlikely that the entire population was converted to Islam to begin with. The remainder of the de-islamisation would have been carried out by Byzantine missionaries.