Dynamics AX 2012 form grid modification

I am learning AX 2012, need some implementation procedural help to modify following changes.

Requirements:

Ø  The field should automatically populate with the same qty as the standard PO qty field.

Ø  The “Original PO Qty” field needs to show on the Purchase Order form screen right next to the Quantity field (like screen shot below).

Ø  Users should NOT be allowed to edit the “Original PO Qty” field on the form.

Ø  The “Original PO Qty” field should continue to be updated up until the time it is a confirmed Purchase Order status.  Then it should lock in the qty and should maintain the original entered amount and not update.

How can I achieve this in AX 2012.

Need your help, thankx in advance.

Salman