Changing default parser in ArcGIS Field Calculator to Python?

I do not think you will find a way to do this because there is an existing, not yet Under Consideration, ArcGIS Idea to have the 'Field Calculator default to Python'.

Esri is a little stuck with this because they want to preserve forward compatibility of old Python code people depend on. I really wish they would have changed the default by now though, as we get a lot of queries from users why their scripts fail in background processing -- because the default (VB) is not supported by arcpy 64 bit!

Here's an example wrapper function to use an alternate default parser. For new code, "PYTHON_9.3", not "PYTHON" is arguably the best parser to use in most situations.

import arcpy
def CalculateField2(in_table, field, expression, expression_type="PYTHON_9.3", code_block=None):
    """Calculate Field using PYTHON_9.3 parser as default instead of VB"""
    arcpy.CalculateField_management(in_table, field, expression, expression_type, code_block)