Depends how far you go back. The Attic, Doric, and Ionian dialects of Greek of Homer's day are very different, though not too far removed from Attic of the 5th c. BCE of Plato's day. Hellenistic or Koine Greek of Megas Alexander changed some again, while the end of the Koine period (~450 CE) changed it some more, though as you get towards the end of the Koine period, it resembles modern Greek. Medieval Greek (450 to ~1453) is basically the same as modern Greek.
I love this idea.
Forgot to ask, but how different are modern and classical Greek? Can you understand one if you understand the other?
Depends how far you go back. The Attic, Doric, and Ionian dialects of Greek of Homer's day are very different, though not too far removed from Attic of the 5th c. BCE of Plato's day. Hellenistic or Koine Greek of Megas Alexander changed some again, while the end of the Koine period (~450 CE) changed it some more, though as you get towards the end of the Koine period, it resembles modern Greek. Medieval Greek (450 to ~1453) is basically the same as modern Greek.