Revision level changes

I am a new Solid works 2008  user and I just finished atoplevel assy. In my assembly I have a hybrid of parts thatwerecreated in context and created as separate parts and insertedintothe assembly with mates. All of the underlying parts have theirowndetailed drawings at rev level X, also the top level assy is atrevX. Knowing that in the future the parts may have to change, howdoI change the part and give it a new rev. but keep the old partatthe old rev? Any help would be appreciated.SolidworksAssemblies