A Python (ArcPy) script to copy values from one table to another.

I need a python script (that can be made into a ArcGIS desktop tool) that will copy values from one file to another file if they are different. Arc GIS 10.3 python script/tool that will allow User to choose a feature class as the main feature class (FCA) User then chooses another feature class feature class (FCB) FCA and FCB share several fields and have a common field with a unique id, but not all fields in FCB will be in FCA and likewise. The user chooses which fields from FCA they want updated The user chooses which fields from FCB that will map to FCA The tool will then compare the value for FCA and FCB and if they are different it will copy the value from FCB into FCA example FCA OBID INCIDID STATUS CLOSEDATE 001 1234 OPEN <NULL> 002 1333 OPEN <NULL> FCB OBID INCIDID STATUS CLOSEDATE 008 1234 CLOSED 01/01/2016 009 1333 MCSC 01/02/2016 the tool will be run and now in FCA it will look like OBID INCIDID STATUS CLOSEDATE 001 1234 CLOSED 01/01/2016 002 1333 MCSC 01/12/2016