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?

>>1
More like Latin > Romance languages.
Learning one should help you to learn another, but they aren't mutually comprehensible, different phono, grammar and lots of vocabulary.