Dispaly mode to Editable mode by using programming in BAdi

Hi All,

When i am entering Line Item Product ID(Which is BOM type), then the  child values are displayed in the below line item.

My issue is : Product ID went to display mode. I am not able to change the field value from existed one(ZSAB) to new one(ZTAB). How to change the field value which is in display mode???

  My ultimate task is changing the Product ID field value from ZSAB to ZTAB in GUI(using ABAP code in BAdi).

Thanks,

Shyam K Gangisetti.