If by "fresnel lens behind the ground glass" you mean that the Fresnel lens is between the ground glass and the photographer, and not between the lens and the ground glass, then you can remove the Fresnel lens without effecting the focus. The image forms at the ground surface, and optics on the photographer side of the ground glass don't change this. You can switch between different brands of regular ground glass by simply laying the new ground glass on top of the existing metal strips. In both cases the ground surface will be positioned in the same place, and that is what matters.
Some advanced screen are more complicated because they have a built in Fresnel. The Boss Screen is more complicated because the image forms inside the sandwich. Also more complicated is the situation where a Fresnel lens is placed in front of the ground glass, on the side that faces the taking lens. As long as none of these are involved, either before or after, swapping ground glasses should be easy. Of course, a photographic test wouldn't be a bad idea.
On your TK45S, the four metal strips aren't really shims. As Peter says, the metal strips actually sit on top of screws, so if necessary, the ground glass position can be adjusted by turning the screws. The was a longer discussion at
http://www.largeformatphotography.info/lfforum/topic/501030.html.