Hi Wang ,
For enhanceing the standard functinality of sap , we have exits,BADI'S,Enhancement points/spots are
available.
First need to find out the Exact Badi - with in that suitable method , In that method need to write the logic related to changed planed order data .
if you want to store it in flat file then call the related BAPI so that all the information related to chaned planned order it will sit in the corresponding tables.
Or
You already find the userexit right .
Create one include in that userexit. and do the same as mentiond above.
Both cased will work if find the exact badi,userexit,bapi.
Hope useful,
Regards,
Sudheer.