requirement class MRP control ?

Hi Gurus,

i have created a material with strategy 40. first i created sales orders without changing any option in Requirement class. then i run MRP. planned orders are created for sales order.

after that i changed the requirement class as No MRP and creatred new sales orders when i run MRP The sales orders which i created after changing the Requirement class is getting added to the planned order.

as the second sales orders should not consider in MRP Why they are gettind added to the planned order.is there any other control that controls the sales order MRP consideration.

Thanks&Regards,
Venkat.