The engine is being completely rewritten, just piece-by-piece. Currently, the renderer, shader system, and game code are completely new. The cgame and network code have been removed. The animation system is currently being written from the ground up. The sound system and UI are still in place, but will be rewritten by Graeme and Robert when they are finished with the mission pack. At some point along the way, John will probably write a new network system.Hey Jim, come post around these parts while you're at it :P
So, even though we are not starting out with a blank sheet of paper, the final result will be a full rewrite.
New Doom Engine Info