Ancient Greek => Modern Greek

If I know Ancient (Hellenistic) Greek how difficult would it be to make the jump to modern Greek?
I know that the phonology is vastly different, but what about the grammar and vocabulary? Is it like the difference between Latin and modern Romance languages, or more like the difference between old and modern English?

>>3
Indeed, Latin helps immensely with French and more erudite English words.
So it's quite a big jump and there's probably no point in trying to learn Modern Greek?