Organizational Policy Update Inquire

Hello, I’m reaching out to you to enquire about the rules in updating/changing an organizational policy. Our company is planning to update our organizational policy with regards to employee benefits during legislated leaves (maternity/parental/sick etc.) to specify that the employees will be responsible to satisfy their portion of the health benefit premium during prolonged absence. We are not sure if the new policy can be applied to those of our “on leave” employees who went on their leaves when the old policy was in effect. I would appreciate any suggestion. Best regards