Can a Holiday policy reduce the number of Holidays for some employees while granting to most?

Company policy is 12 paid holidays, received only if scheduled to work those days. Some workers required to work weekends. New policy makes most Holidays fall on Mondays. E.G. Workers who don't work on Mondays or other designated days will lose the benefit which all other employees who work M-F continue to receive. There are no pay adjustments for Saturday or Sunday workers. Many shifts are 10 hour/4 day shifts. This reduction in a stated benefit is selectively reduced for about 20 % of the workforce. Does reducing pay by 5 or 6 days violate retirement earnings or violate any tax code when applied to a limited number of employees while granting the full benefit to most?