How should I handle students who complain they were "downgraded" or “upleveled” because of the renaming?

You can reassure them that:  

"You have not been downgraded nor upleveled. We added a new course below your current level to better support students with no English prior knowledge. Your achievements are fully recognized, and you are on the right path to continue progressing."

If you still need help, don't hesitate to write for us on support@slangapp.com.