Hi Charles!
The Account Group is the essential controlling element in the ERP system. In CRM there is no such concept of the account group that controls all important functions as is the case in ERP. Therefore there is no 1:1 mapping of the account group between ERP and CRM. The account group is mapped to the sales classification in CRM. This allows you to specify which sales classification the business partner receives depending on the account group maintained in ERP. This is why you see the change in table CRMM_BUT_FRG0041. You must specify a numbering group for which the new business partners are assigned their new BP ID in CRM. This is what the BU_GROUP field refers to in BUT000, the numbering group and not the account group. So a change in account group will never change the ID of the BP in CRM. Changing the ID of a BP in CRM can have serious inconsistencies. Note 504265 provides details on this and a possible work around. Hope this helps!
Conall.