How will changing section rosters affect student grades and teacher gradebooks?

Course grades are tied to the course and the student, so as long as your students are staying in same courses, their courses grades will still be associated with them regardless of any section changes. The same goes for teachers. As long as the teacher is still the teacher of record for the course and all assessments are still tied to the course, those assessment will display in the teacher's gradebook regardless of any section changes.