Actually it was only one engine switch (Quake 2 to Unreal tech, keeping content) and one restart (in 2004 IIRQ) where they dropped everything and started again. Not sure how much of the engine survived the restart but I suspect at least the renderer was updated. They did switch physics engine rather recently though, but that doesn't really have any impact on the game as such. It's just a matter of hooking it up to the existing system and off it goes. 
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