BOM changes(Not creation) with EC Number

Hi

I want to make the ECN as mandatory while changing the BOM,not for creation.

I tried by setting OS27,OS25 settings for BOM. I tried BOM creation with status 3(With history requirement). System gives only warning message,not the error message.(Message no. 29045)

I tried to make the Change number field as mandatory,but it applies it to CS01 & CS02. I want only in CS02.

Plse help me out if any knows the soultion.