Item category assignment for source account &1 is changed from &3 to &4

What causes this issue?

During the chart of accounts conversion, the system tries to merge
accounts into the new account (&V2&) that have different item
categories. As a consequence, the assignment of the item category in the
Customizing settings for document splitting changes.

System Response

The system already contains transaction data that was posted based on
the old settings (&V1& with line type &V3&). The change can be
implemented either by reassignment or by a change or by activating or
deactivating the indicator for overwriting.
Note that the change to the Customizing settings takes effect from the
conversion date and applies exclusively to newly posted documents.
Existing documents are not converted accordingly.
When new documents are posted, a change to the item category assignment
may lead to different and possibly incorrect splitting results or
posting items in the G/L view of the FI documents. This may in turn
result in incorrect segment reporting for the related accounts.
In some cases, you may get an error message when trying to enter
documents that were posted before the change. For some document items,
it is no longer possible to obtain necessary account assignment
information due to missing Customizing settings. This happens
particularly if the check for required entry fields is active for one or
more splitting characteristics in the system. In these cases, an
incorrect change to the item category may result in a system downtime.
In addition, documents that were posted before and after the change,
respectively, may not be comparable due to different splitting results.
In general, the problems may occur in the following situations:
If a change to the item category results in a change to usage in the
active set of rules for document splitting. The following cases may be
relevant:
Usage as item category of the leading item
Usage in the fixed value (processing category 0)
Usage as item category specified (processing category 1)
Usage as base item category
Usage in splitting based on current account balance (processing category
2)
If a change to the item category leads to the account being excluded
from a business transaction variant.

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

How can this happen?

If you use standard splitting methods with the standard set of rules,
changes to the item category may be critical, for example, in the
following cases:
Business transaction 1000 (payment): Item category 04000 (cash account)
is changed to 01000 (balance sheet account):
Usage of the account changes from "item category to be processed" to
"basis item category". Result: This account is no longer posted to by
document splitting.
Usage of the account changes "item category of the leading item".
Result: When cross-company code payments take place, the system cannot
determine the company code for the leading item, and it is no longer
possible to use the account assignments at cross-company code level.
The business transaction variant needs to have at least one item with
item category 04000. Result: If there are no other line items with
category 04000 (cash account), the document cannot be posted.

How to fix this error?

Make sure that the new assignment of the item category (&V4&) and the
combination of the accounts to be merged are correct. Check any existing
Customizing settings in the system as well as stored splitting rules
with regard to the new item category assignment and make any required
changes. In addition, an extensive test of all existing business
processes must take place.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.