Changes in PIRs

Hi,

Kindly let know of until which point in the planning process can PIRs be changed. Consider that PIRs had been entered thru MD61, MRP run and Planned Orders generated. How r changes to the PIRs be taken into effect then? If a planned order has been converted to a production order and now if a change is required in the PIRs entered how can this be effected? Will the system take the changes to the PIRs into consideration even after a production order has been generated corresponding to it?

thanks in advance,

Deepak