Maya set shading node classification on existing nodes like `maya.cmds.shadingNode` does for creation of new nodes

maya_asShadingNode.py

from maya import cmds


def asShadingNode(node, **kwargs):
    """Set shading node classification on existing nodes.
    
    Like `maya.cmds.shadingNode` does on creation of nodes
    this command can add or remove a shading node classification.
    
    It matches the same argument names of `maya.cmds.shadingNode`
    like `asShader` and `asLight`.
    
    Examples:
        # Add asShader classification
        >>> asShadingNode(node, asShader=True)        
        # Remove asShader classification
        >>> asShadingNode(node, asShader=False)
    
    """   
    mapping = {
        "asShader": ":defaultShaderList1.s",
        "asTexture": ":defaultTextureList1.tx",
        "asLight": ":lightList1.l",
        "asPostProcess": ":postProcessList1.p",
        "asUtility": ":defaultRenderUtilityList1.u",
        "asRendering": ":defaultRenderingList1.r"
    }
    for key, value in kwargs.items():
        if key not in mapping:
            raise NotImplementedError("Key not supported for shading node type: {}".format(key))
            
        dest = mapping[key]
        src = node + ".msg"
        if value:
            cmds.connectAttr(src, dest, nextAvailable=True)
        else:
            cmds.disconnectAttr(src, dest, nextAvailable=True)
                
                
# Using shading node directly on creation
cmds.shadingNode("blinn", asShader=True)
  
# The equivalent on an existing node using this function              
material = cmds.createNode("blinn")
asShadingNode(material, asShader=True)

# Or remove the classification on an existing node
asShadingNode(material, asShader=False)

The equivalent of this could actually be achieved by regularly using the maya.cmds.shadingNode command but passing it the shared=True flag. As such the above code is redundant.

However, you'll need to still pass it the node type explicitly. For example to mark it as a texture.

maya.cmds.shadingNode(node_type, shared=True, asTexture=True, name=node_name)

More details on the Shared flag in the maya documentation