PTO: Vacation accrual overwrite eligibility rule - the best practice

   I would like to find out what would the best practice for override vacation accrual rules for different situations.

For example our accrual bands are 0-5 2 weeks, 5-10 3 weeks and 10-99 4 weeks. We have several employees who get negotiated vacation of 3 weeks from the beginning of their employment. So they start accruing 3 weks from day one. For this we are using continuous service date ( 5 years prior to their hire date) but because of this date when the employee completes 5 years of service they automatically start accruing at 4 weeks level which is against our business rule. We need employees to complete 10 years before they accrue 4 weeks.