Can't break external references

Hello,

I am very new to SolidWorks and am literally learning on the flywith no formal training. So if my descriptions or terminology iswrong, I apologize as I hope to communicate effectively!

My problem is:

I have created a basic assembly from several different part filesthat have been mated together along a common axis. I've createdthis assembly with some initial trial and error, and have used theprocess for quite a few assemblies to create drawings. All otherassemblies and the drawings I have created from them have workedout fine. The assembly consists of several sections of a shaft, ofvarying lengths and diameters. To create a "new" assembly, I firstsaved the original assembly as a copy. I then renamed it to the newfile name. Working with that new file, I would like to add, forexample, an extension to the end of the shaft. It seems easyenough. The process I use is to click on the component, editcomponent and create an extruded boss. I am aware that it actuallychanges the original referenced part, although not currentlyopened, adding the new boss. This is normally fine, but I want tocreate multiple assemblies using the basic parts. Problem is, if Iclick the "No external references" button, it still modifies theoriginal referenced parts. This of course screws up my otherassembles using these same part files. SolidWorks help states thatI can break or un-lock external references by simply right-clickingon the individual component in the design tree, but I can't seethat option available to me. I simply want to add new features tomy assembly WITHOUT changing anything outside of the currentassembly. I appreciate any help with this seemingly simple, buttroublesome problem. Thank you.SolidworksAssemblies