Hello Venkat,
The system behaviour is correct. You cannot change the consumption behaviour for an ID if already business transactions are present for the combination valuation area, company code and product type. What you can do is create a new product type based on 02A and possibly do a Sec account transfer , if there are Live deals present. The system will not allow to change from weighted average to Single position Management , when deals are already present for a product type. Or may be try corporate actions to transfer positions from Old ID to New ID .
Regards,
nikhil