Saul

This is exactly the problem 3d coat has, one "easy" solution would be the posibility of subdivide going up and down in retopo room. Right now you can subdivide de geometry doing snap with the high res mesh, but you cannot go down into a lower res. I think this is a must, even only for retopo, because you are not completely sure the retopo mesh is going to produce the result you want when the render subdivide the geometry at render time.