You want to generate a text file, JSON or other format, to list the required variable changes for a certain build number or new build.
Run PS to pull in the new vars, compare to the last build and add those that are required for a new build.
Steps to produce desired actions
#1. Development team will create a JSON file for the required variables to be changed, add new variables, and remove those that are no longer required.
#2. PS script to modify TFS build.