For now, we recommend you avoid changing grading period or school year dates and parameters after their creation. Gradebook was not designed with this type of flexibility, so changing the dates of a term after assignments exist in that term will force the assignments to be past due at the new term's date and unassignable beyond their original due date. If students have already enrolled, their future term enrollment may be removed or changed. There may be problems with new student enrollment as well, or the auto-enrollment process that moves students into future grading periods.
We're currently looking into this issue with our development team.