13th month bonus is not really combined into monthly pay, but the variable/performance bonus got reduced while increasing your base salary by up to a 13th month equivalent salary, so you get lesser bonus with the new pay scheme, but overall similar annual compensation compared to before.

Its good for older staff because they got the adjustment, but newer staff who joined after the adjustment will only get pre adjusted salary with lesser bonus. Hence newer staff will have lower annual compensation for the same YOE/job scope.