How do we revise our PTO payout policy in midstream?

Q: "Over the past few months we have had several employees voluntarily leave our employment. According to Oklahoma state law, we are not obligated to pay the employee for unused PTO hours; however, according to our employee handbook, the employee will be paid for unused PTO hours. Can we change our employee handbook, with proper notice to all employees, to state that the company is not obligated to pay resigned employees for unused PTO hours? Currently employees accumulate PTO hours on a yearly basis. Can we change our PTO policy to allow employees to accrue on a specified basis, i.e. hourly, daily, monthly instead?" – M., Oklahoma