varient classes for sales order

Hi,

Mat A is a configurable mtl. Assume class C (300 class type) is assigned to the material in material master & in configuration profile.Sales orders are presently created using class C.

Now I want use another class --D for the same material for creating sales order. I have assigned the class D in material master & configuration profile with the standard class indicator marked as on.

But system is proposing the Class C only during sales order configuration???

Can anybody guide me on this???? Can I delete the assignment of the previous class C???

Regards,

Manish