I don't think arcpy has the capability to change symbology (other than importing from an existing layer file).
You can use the comtypes module to access ArcObjects from Python. That will allow you to control symbology.
Have a look here for more information on ArcObjects and Python.
Python and ArcObjects
The snippits.py has some useful examples.
You will also probably need to look over the ArcObject help and Object Model Diagrams. Carto is the one which will have the symbology.
Ben