SAP Note 23370 - Cannot change base unit of measure in APLA

Symptom:

The system rejects any attempt to change the base unit of measure when a routing exists, even when the routing has been deleted.

Cause and prerequisites

The assignment of the routing to the material master is retained despite the deletion, in order to enable changes to be traced.

Solution
This functionality is not supported. The routing must first be archived.
Once no valid assignments exist between the routing and the material, you can change the material master. =>
1.) Delete all material-routing assignments
2.) Run the routing archiving program

Further, the problem can be avoided by entering a new material number.