Is there a way to swap out a subassembly or should I just rebuild the whole assembly? I made the assembly with my pavement section, inslopes, ditch, and back slope. Then when I was looking through the subassemblies I found a pavement section that would work better. Can I swap it out or do I have to rebuild my whole assembly?
Yes, what you do is, for the basic case, you find the sub assembly that you now have decided works for you on the palette.
You Rt-Click and Select, Apply Tool Properties to SubAssembly, then Click on the sub Assembly component you wanted to swap out.
For a 'global' change to a corridor pavement section, one would want to place a modified version of the Sub Assembly(s) in question on a User palette, and alter the properties. Then use the technique above to swap out the undesirable sub assembly for the modified one.
Or apply your changes to the sub assembly(s) in the Prospector View of the Toolspace, expand the sub assembly collection and edit their properties from there.