Foreign key &-& can only be changed in the referenced type

What causes this issue?

You wanted to create or change a foreign key for an expanded field.

System Response

The foreign key for an expanded field is only displayed, but cannot be
changed.

How to fix this error?

Change the foreign key in the referenced type itself. The change also
affects the structures or tables that use this type.

Error message extract from SAP system. Copyright SAP SE.