Vacation pay effective date

We currently have a group plan DBA table setup for vacation pay that pays increased % once a certain # of months threshold is met (based on start date).  If the employee meets the criteria in the current pay period, the vacation % is then increased going forward.  So, for example, if the employee were to reach the 5 year tier in the first pay March, they would move to the next, higher tier in the following pay.  If we were to move to a different compensation method of, in the calendar year in which the employee reaches their next