You can reduce the number of levels in a subdivision surface using the Collapse Hierarchy feature. Collapse Hierarchy maintains the original shape of the surface, but moves the subdivision edits from finer levels to coarser levels, making the coarser levels more dense as a result.
For example, you may want to make a topology change to a face that resides at a finer level using the . Topology changes such as this can only be made at the base level (level 0) of a mesh, so collapsing the hierarchy so the finer levels of detail appear at level 0 on the mesh would allow you to switch to Polygon Proxy mode, and make the edit at the base level.
To collapse subdivision levels
For example, if you collapse two levels, previous level 2 vertices become level 0 vertices, level 3 vertices become level 1 vertices, and so on.