Plan eligibility based on plan balance

Summary:

Hi Everyone,

We have a requirement where employee is eligible for a PTO plan based on the grade. Ex: if he moves from Grade 4 to 5 employee will be eligible for unlimited PTO and existing PTO plan will be end dated. this is straight forward. Our requirement is if employee got negative balance in the current PTO plan, even though he moved to from Grade 4 to 5, they should stay in PTO plan till they accrue the balance to make it positive. How we can design this; how can we keep the employee stay in the plan the balance is