I am making some assemblies where there are some components that remain constant and would like to reuse and others that I would like to do a save as along with the assembly (so I can modify them). 

Assembly 1

     Part 1

     Part 2

     Part 3

Goes to

Assembly 2

     Part 1

     Part 2

     Part 4

I have tried to do a pack and go, and where as it does create a new Part 4 and Assembly 2, Part 3 is still in the assembly and I have to manually replace it with Part 4.  I would like to skip the manually adding if possible.