Hi Yogi,
Please go through the below menu.
SPRO->SAP - Implementation Guide->Cross-Application Components->SAP Business Partner->Business Partner->Basic Settings->Identification Numbers->Define Identification Categories
Identification Categories( CRM004 ) , remove the tick in ID Unique and display only column and then check in tcode BP , hope it will allow to change.
Let us know if issue persist.
Note:
The menu path has given of older version 7.0.Please try the Identification Categorie if you are in new version.
Thanks,
Ajit