An amendment to the Employment Ordinance requiring that workers' commission payments be included in the calculation of sick leave and holiday pay took effect yesterday. The legislation, which some employers have said will add greatly to their costs, has been blamed for a recent surge in requests by employers to staff to ditch employment contracts and become self-employed. The city's two biggest property agencies, Centaline and Midland Holdings, which employ more than 6,000 agents whose earnings are made mostly from commissions, said they had no plan to require staff to make the switch.