- Click on open file and select the new drawing.
- Don't open the drawing but click on references.
- In the pop up window double click on the part/assembly name and browse to the new file.
- Click OK and now open the drawing.
- The new drawing should take reference from new file.
You can save yourself some work by changing the way you make your changes. When you want to copy a 3D part to a new file and also change the drawing to a new file, open up both the part and the drawing. Make your change to the part and save it. This will update the reference in the drawing to the new part. Now save the drawing to the new name and you are done. If you want the new part to change in any assemblies that the old part was in, then you should also have the assemblies open when you change the part to the new one.