Change of account group not possible; customer &1 used in SD documents

What causes this issue?

Changing the account group is not possible for customer &V1&. Customers
of account group &V2& are not permitted to use partner role &V3&.
However, there are still SD documents in the system for the partner
role and the customer. Changing the account group potentially means
that these documents are not processed.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Choose an account group in which customers can use partner role &V3& or
archive the docunents for the customers.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.