As stated in 3.2.4 this is not allowed for Enumeration DataType. Ideally, an approach as described in 7.12 had been used allowing the extension. If not, the only option is to deprecate the old Enumeration (and all its usage in Variables, Method Signatures, Structured DataTypes etc.) and define a new Enumeration (or preferable an extensible approach). This is very cumbersome, specifically if the DataType has been used directly or indirectly in various places.