VBCS Rule: Force LOV value to change when another field changes (overwrite existing selection)

Summary:

I have a LOV field called Regulated Position. When Legal Entity (BusinessUnitId) is X, I need Regulated Position to be automatically set to LOV value = 1 and become read-only.

This works only when Regulated Position is empty. If the user first selects another LOV value (e.g., “2”) and then changes Legal Entity to NUInvest, the field becomes read-only but keeps the previous value and does not update to “1”.

I already tried a Business Rule with Set Field Value / advanced expression return '1'; and confirmed the rule condition is re-evaluated when Legal Entity changes, but it still doesn’t overwrite an existing value.