Sales Item Cost

Dear ,

i use JDE 9.1.5.

i have the below scenario:

1. item XX cost 50

2. created many sales orders and sales updated.

3.after one month i discover the cost for the item XX is wrong and should be 20

4.i changed item cost by P4105.

5. what about sales orders which already updated and close what the proper way to fix the cost in these sales orders?

Thanks,