Interesting question, Miriam.
No, I don't know about any report. What I do know is that Moodle has something called "Grade History". I have never really understood what this is and how it is used. If you search your moodle administration settings for "history" you will see some settings about grade history.
Its name (Grade history) seems to imply what you are looking for. Note that it must be turned on.
So, maybe moodle keeps something about grade histories in its database, but you will need to write some SQL to extract exactly what you want.
As I searched around a little, it appears that there might be some plugins about grade history.
Well, maybe others will be able to jump in and help you figure this out. I just wanted to point in to the general direction.