I had a similar problem this past semester. I hired an assistant from one of my courses. She remained in the course as a student, but needed to help me input grades for her own class.
What we discovered was that she could be assigned a teacher role and still add grades to her name, along with everyone else. (In fact, for some reason, even my name shows up in the gradebook and it's possible to assign grades to me.)
Then, at the end of the semester, we needed to have her included as a student in order to properly calculate the weighted and curved grades for the class. We removed her from the teacher list, added her back to the student list, and fortunately, her grades hadn't disappeared.
I'm just posting this so that if anyone else has the same trouble, you don't have to wonder whether you will lose the grades you entered when you make the role switch.
Ideally, there would be some kind of customizable third/dual role that allows for my situation as well as the graduate assistant situation described above. Another situation is when I want to let my colleagues "lurk" in my courses to learn about how Moodle works. I give them teacher status so they can see the gradebook, etc. When I give them a teacher role, they show up under the list of teachers visible to students.