Template Design: Use a switch to turn off an element and control parametric constraints.

I am working on some alternatives for a bridge design and I want to be able to quickly switch what the outside railing looks like. I am looking at PPM1: Figure 4.4.2 and I want to quickly be able to switch from a 32" railing with bullet rail to a 42" railing without bullet rail. 

I can use my switch to turn off the rail, but I also want that same switch to change the width of the railing to 1'-3". I have a parametric constraint set up for the railing width, but I am not sure if I can create a rule that applies to a parametric constraint.

Please let me know if anyone has any ideas.