by Per Ekström.
Yes, this is a very real use case, but, I'd like to make the argument that you could solve that problem without adding retroactivity. I believe that if you add in retroactivity, you're opening a whole can of worms best left unopened.
In this particular case I'd add in a tool to manually adjust a grade, which sends out a mandatory notice to the student that his grade has been changed for a certain reason. Of course, that's just my solution to it, others are certainly possible.