Customer & active in customer hierarchy: Inconsistencies are possible

What causes this issue?

The customer is active in one or more customer hierarchies. By changing
the (fixed) hierarchy level allocation, inconsistencies can occur
within the hierarchy/hierarchies.
Example: Customer A with level allocation 01 is used in a hierarchy as
follows: Customer A / Level 01
|
|--> Customer B / Level 02
If the level allocation is now changed from 01 to 03 within customer
A's master data, then the hierarchy is inconsistent since level 03 is
then above level 02.

How to fix this error?

Only change the level allocation of a customer if you are absolutely
sure that the change does not generate any errors in the hierarchies.

Error message extract from SAP system. Copyright SAP SE.