Question about character rigs and substitutions.

Hi,

I have fully rigged my character. At the moment he is in a forward facing stance towards the camera. I understand substations with things like hands etc but what if I had something more complex?

Say I wanted to switch the head only, from one view to another that had multiple children associated with it, would that be easy enough to do as a substitution and if so how would I go about it?

Or say I wanted to switch his entire body from one view to say a side view that had a separate rig setup for it, would that be a substitution or would I just import that from the library?

Thanks.