This topic explains why and how one can change the account group for a customer.
If, for example, a customer who has always fulfilled the function of a payer then takes on the role of a sold-to party, you have to assign the new function to the customer. However, since screen and field selection in the customer master record are controlled by the account group, you can only assign the other function by changing the account group.
Changes to the account group and the accompanying partner functions can only be made from a lower level to a higher level. For example, this means that a sold-to party cannot be assigned the function of a payer as fields which have already been maintained for this sold-to party would have to be masked. However, you can assign the sold-to party function to a payer.