Hi Marc,

> but for example sometimes I find I need sub-polygon
> displacement on certain parts and I haven't planned
> at the start.

Could you maybe show an example so I could understand what kind of displacement you're talking about? Do you mean something like painting on some ridges and raised areas?

One method that can work well for that is to import your model into a brush based displacement program - 3D-coat has a voxel method for sculpting like this, which is independent from the initial mesh topology since it converts from a mesh form into voxel form for doing the actual sculpting.

- Michael