Koine is the evolution of Attic Greek; it features somewhat simplified grammar and syntax and it ends up being pronounced a lot different than Attic. The modern Greek pronunciation is very close if not identical to what the Koine ended up sounding like. The orthography didn't change though and hasn't changed to this day.

Doric Greek and Attic Greek are two dialects of the same language; they are mutually intelligible with some effort, think of the relation like that between Geordi and Cockney English, both evolving from a common ancestral language. If you want specifics of the phonetic correspondences and grammatical variations that would get too technical indeed, if you don't have some knowledge of Attic at least (dunno if you do).