Change configurable link of material BOM

Dear ALL EXPERT,

We have a semi-finished material which is linked with a configurable material(Plant specific configuration).

From CS12 it was linked with configurable BOM.

An example:

Configurable material: GAA000

Semi finished material: GBH015

BOM: GBH015

Item: 10 = Y-SMPL

Item: 20 = YAB01

Item: 30 = YAC01

IN BOM stages for GBH015 we need to change the item 10. It will replace with YAA01.

In CS02 it is showing "BOM can only be change via standard product GAA000"

In CS23 it is showing "No selection made for usage of material GBH015"

Can anybody advice me?

Thanks and B/R

Bishnu