Instead of renaming the copy of the drawing, it might be better to rename it by doing pack and go and thus make a copy of the plan and the part and then you make the modifications you need.
A hack to help you out, the original part either you change the name or you move it to another folder. You open your copy of the plan and there normally you should have a massage of the type blah blah blah I can't find the piece do you want to look for it there you answer yes and you go look for the copy of the piece.
Finally, don't forget to go back with the original part.
The message is pretty self-explanatory and MaD is right: you're looking to replace a part with an assembly, which SW doesn't know how to do by the way you go.
I think like the others that it is indeed an attempt to replace a part by an assembly or vice versa (I have rarely seen SW fail at this level).
On the other hand there is a flaw to exploit, from memory I had managed to change a part model by an assembly by doing the manipulation of replacing the model directly in the view (right click, replace the model).