Changes in process flow

Hi All,

I have an requirement below..Please help.

At present, We are following this process that only after release, the PO can be printed and send by email to the Vendor.  Once released, the PO cannot be changed until cancel released with ME28.

Now, We need to change the process that even before release, the PO can also be printed and send by email to the Vendor. Once the acknowledgement details are received from the Vendor, the same will be updated in the PO and the PO can be finally released. 

Thanks