In ZBrush tools can’t be retopologized without destroying existing 3D layers. In Blender, objects can be retopologized and existing Shape keys will be altered accordingly. The altered base object and its shape keys can then be reimported into ZBrush. The conversion is tedious work.
DynaMesh is a new feature Of ZBrush 4 release 2 that allows the user to add to a mesh while at the same time maintaining an evenly distributed topology! What is even better about DynaMesh is that the new topology is mostly made of quads, making it easier to sculpt in ZBrush and animate in other 3D apps.