This is a package of RELAX-NG compact syntax schemas for editing and validating instances of UBL 2.0 and draft UBL 2.1 schemas.

This uses James Clark's Trang utility plus some massaging:  the presence of the optional UBLVersionID attribute is obliged to have the value "2.0" or "2.1", which then in turn ensures the remainder of the instance conforms to that version of the document model.  The documentation illustrates a simple example of this.  Your editing environment points to a single schema and all versions of that document type are automatically accommodated accordingly.