Switching and Cutting Video and Audio Items Separately
You should be able to drag the original clip from the Browser into the timeline and replace the clip you're working with.

If you want to make sure not to mess up your edit, double-click the 'broken' clip in the timeline to bring it into the viewer, and make note of your in and out points.  Then open the original clip from the browser into the viewer and set the same in and out points. Now replace the 'broken' clip in the timeline with the new one from the viewer.

If you're asking how to switch audio and video independently without breaking the relationship, I'm not sure.