Benefit Start based on Adjusted Start Date instead of Hire Date for Old Employees

Inviting Inputs from Experts on Business Requirement where selected population of a firm can have benefit rules are expected to operate based on Adjusted Hire Date/Adjusted Start Date Attribute instead of Hire Date. ( Difference between this set of population is the former is hired before specific date + On certain Grade ) Please suggest levels at which this bifurcation can be best set. Impact on 2 Benefit Plans out of 28 Plans in total and corresponding Life Events where data changes are using Hire Dates