I don't see why there would be any differences in the files between the lifetimer and Steam versions. I believe you could just rename your UrW directory to something else and then reinstall UrW (or copy the directory and "repair" it) to get hold of the proper vanilla version of the file.
I'm following the lifetimer rather than the Steam route (typically using alternatives to Steam whenever there is one), so I guess you might not be interested in my copy anyway, in case it might be different for some reason.