MRP Re-order point qty change -Old PR's not deleted

Dear Guru's,

I have uploaded new safety stock settings yesterday for MRP area A420_Z200, however, the old PRs created from previous MRP run were not deleted , some PRs created in 2011, 2012 are still shown in Open PR list; from understanding, those PR should be deleted when new MRP runs, new PRs should be generated based on safety stock settings and material requirement list.

1.Could you pls let me know the reason like why the old PR's are not deleted.

2.How to clear the these PR's in ME5A report

Changes done: Reorder point Qty is changed from Qty: 4 to Qty:3

Please advice the cause.

Thanks in advance.