Hi, 
If I retoplogize my character halfway then continue to sculpt and finish the head, can I retopologize the head only? I wouldn’t want to redo the whole figure.
yep.
work your mesh for the topo as you see fit.
when you’re good with that, then take the mesh and create an adaptive skin. export out that mesh (or keep the lowest subD during sculpting).
once you’re done with all that jazz and want to finish the topo for the head you’ll need to load that mesh into the topology menu…there is a button for “select mesh” or something like that. When you’re ready to start the topo for the head just import in the mesh that will be the base topo and select that mesh once you’re into the topo tools. then just go into edit and continue the topo or change it as you see fit.
careful though…you can crash Zbrush if the selected mesh is over 200k I think…I haven’t tried it oin 3.5 yet, but I believe that was the number that would crash it in 3.1