A shading group is needed for geometry to be renderable, hence the reason the lambert1 cannot be deleted. Assign it to the object, then you can delete whatever shading group you made. Hypershade makes shader swapping easy, simply right click the shader you want to delete and choose 'select objects with material', then right click the shader you want to assign and select 'assign material to selection'.
Hi there thanks for the reply. I followed these steps but when I press delete it deletes the whole mesh.