^No, it's earlier:Pope St. Athanasius in his Life of St. Anthony the Great notes that 1) St. Anthony was converted (in the sense of to the monastic life. He was born into a Christian family) by hearing the Gospel read in the DL 2) he spoke no Greek, just Coptic.
St. Anthony had that epiphany around 285, so by then Coptic must have been used even in Lower Egypt (i.e. the Nile Delta; Upper Egypt was more remote and Coptic reigned there, lingering into the modern age). When it was switched to Coptic in Alexandria, a heavily Greek city, is another question. There may have been some use of Coptic early on, but it seems that the switch to totally Coptic didn't happen until after the Arab conquest.