Modify > Convert > NURBS to Subdiv, Polygons to Subdiv

Both conversion operations (NURBS to Subdiv and Polygons to Subdiv) have the same options in the options window.

Maximum Base Mesh Faces: Set the maximum number of faces the original surface can have to successfully convert it to a subdivision surface.

For example, if the original surface has 900 faces and the Maximum Base Mesh Faces value is 1000, the conversion will succeed, but if the surface has 1200 faces, the conversion will fail.

Depending on the complexity of the original object, the resulting surface may be represented with fewer faces.
Maximum Edges Per Vertex: Set the maximum number of edges each vertex in the original surface can have to successfully convert it to a subdivision surface.
Keep Original: Keeps the original object when creating the subdivision surface. When Keep Original is turned on, you can select the initial editing mode for the subdivision surface. When this option is off, the original object is replaced by the subdivision surface.
Subdivision Surface Mode: These options are available only when Keep Original is turned on. Select the initial editing mode for the subdivision surface.

Hotkeys

` (left single quote beside the 1 key)	Turns on/off the display of the original surface or the converted subdiv surface.
+ `	Displays both the original surface and the converted subdiv surface.
+ `	If a NURBS surface is selected: Modify > Convert > NURBS to Subdiv If a polygonal surface is selected: Modify > Convert > Polygons to Subdiv
+ + `	If a NURBS surface is selected: Modify > Convert > NURBS to Subdiv > If a polygonal surface is selected: Modify > Convert > Polygons to Subdiv >
Page Up	Increases the Display Level of the subdiv surface (when in subdivision surface standard mode).
Page Down	Decreases the Display Level of the subdiv surface (when in subdivision surface standard mode).