Can a company that has 12hr and 8hr employees change overtime rules from low hours which was fair to everyone to 12hrs cover 12hrs and 8hrs cover 8hr

I work in maintenance for 30years and the overtime rules went by hours regardless of shift that way no one could manipulate overtime. Maintenance overtime rules worked so good that the rest of the plant adopted those rules. Now the maintenance department head changed the rules to 12hr employees cover 12hr employees and 8hr employees cover 8hr employees regardless of hours. Now an 8hr employee can work overtime with high hours and get more overtime opportunities than a 12hr employee.