Allocation Table related question IS Retail

[subodhd]

Hi
What I notice is that once we create allocation table using WA01 transaction and save it, the purchasing group field in the header is grayed out and we can't change it in WA02 transaction. Is there any utility or transaction or config setting whereby one could change the purchasing group in the allocation table header after it has been created.(either mass update or individual )
We use IS Retail.

Thanks in advance
SD