how to firm up work order

We are running MRP (P3483). We have work orders entered for 6 months in advance. We want MRP to calculate the component requirements for these work orders and NOT change the dates or quantities on the work orders. How can we do that? Is there a way to indicate these work orders as firm?

Currently, P3483 gives messages changing the dates and quantities of the work orders or cancelling them altogether. We need P3483 to calculate the component requirements only.