As a newbie modeller from Team Avalanche, my feeling is that the originals were, for the most part, not 2D renders of 3d models. My judgement is mostly based on the fact that, when you try to remodel a scene (and we can have direct input regarding walkmesh and camera location), there are always some elements whose perspective is wrong, or have misalignment. These "errors" can hardly be noticed until you make actual 3d representations of them, but there you have it.
Now, as in Team Avalanche we do make 3d environments to make 2d renders, such work can be later re-used within the scope of a remake, in my opinion. However, some scenes may need revision, as I do know that some modellers did the texturing by "applying textures from view", which works when there's only one still image, but is completely wrong if the camera is moved around.