I have multiple surfaces that are created a seperate parts that are then put together in an assembly.  I need the assembly to be able to move the surfaces around and orient them with each other.  In the future this orientation will change and I need the subsequent work ("finish work) on these parts to derive from these changes.

Now that the surfaces are oriented, I need to trim and create solid from these surfaces.  The only way that I can see to trim this is the save as the assembly as a new part (not derived any more) and use the surface trim tools, create solid.  I then do a lot of work (finish work) on the solid to create the final part. 

The problem is that when I make a change in the original assembly, the changes do not propagate to the early save as part and all of my "finish work" redone. 

Is there are way to create a derived surface from the assembly surface parts?

surfaces in assembly:

Must export to trim surfaces to create body: