2to3 inscrutable output

[rurpy at yahoo.com]

What is this output from 2to3 supposed to mean?
  $ cat mysub.py
  isinstance (3, (int,float))
  $ 2to3 -f isinstance mysub.py
  RefactoringTool: No changes to mysub.py
  RefactoringTool: Files that need to be modified:
  RefactoringTool: mysub.py
Why does mysub.py need to be modified, and how?