Question regarding PTO Payout

Hello... Our current PTO policy is to allow full time staff to accumulate up to 720 hours of PTO. After they reach that limit, they are unable to accumulate additional leave time until their balance goes below that cap. Also, if an employee were to leave the organization, they would be paid out at 100% for up to 200 accumulated PTO hours, then at 67% for any leave time above that. We would like to change the current policy to educe the maximum accumulated leave time to 240 hours. Currently we have one staff person with more hours than PTO time than that. My questions are: 1. Can we pay the employee out at 67% for any hours above 240 or must we pay him out at 100% for any leave above 240 hours? 2. Is it legal to pay anything less than 100% regardless of how much time a person has on the books? 3. How much notification must we give staff before putting a new policy in place?