If you are using planning bills and you explode the forecast for a few components. Is there a way to

Imagine you have Car to sell 1,000 units

in the planning BOM you define

CAR

     Blue Car     20%

     Red Car     10%

     White Car     20%

     Black Car     50%

Now you enter a forecast for the item CAR of 1,000 units each month, and you run MRP to explode the forecast. The system will generate the forecast for these items as follows:

     Blue Car     200

     Red Car     100

     White Car   200

     Black Car   500

Now the question is imagine you want to change the forecast instead of 1,000 to 500 when you run the MRP explosion for the forecast the quantities remain in 200,100,200 and 500 and not updated.

How can this be perfromed