Hi SM
Firstly, a customer can only belong to one account group. And you can't change it by extending the customer.
But you can switch the customer from one account group to another, with XD07. Try this, ensuring the number ranges and field selection settings of the two groups are compatible with each other.
In particular, ensure that any fields with values in the old acct group aren't now hidden in the new, as the old value can still be read and used in transaction processing.
Regards