Compliance with the 411(b) accrual rules is a form requirement and is applied prospectively only. Thus, if I am a class A employee, I have a scheduled level accrual for all future years which satisfies the 133 1/3 rule. On the day I become a Class B employee, looking prospectively, I have a new level accrual for all future years which satisfies the 133 1/3 rule. You do not look back after the change in class nor do you assume a change in class prospectively. That is, of course, unless there is a progression from A to B (eg A is less than 5 years service; B is more than 5 years)