Edit graphics only?

[LordZeebmork]

Is there any way to make a PWAD that changes the graphics in levels when it's loaded but doesn't change the levels?

[ReX]

Yes. However, this requires that you replace some or all of the existing graphics from DooM/2 with the new graphics. You can create a PWAD with just the new graphics, and run it like any other PWAD. The original DooM/2 maps will remain unchanged, but the graphics will be different.