While I certainly agree there are many cases where both things happen in the same publication (i.e., the circumscription for the species "bus" changes, at the same time "bus" is moved from the genus "Aus" to the genus "Xus"); I see these as two unrelated things.
I agree in theory. However, in practice, this is hard to distinguish. A clear case ist that a replacement epithet does NOT change circumscription. But we already disagree about merging two species: I don't think is necessarily changes the circumscription, it may only correct a previous oversight (i.e. that one circumscription is a subset of the other). But who in practice establishes whether the larger circumscription is the older or the younger name.
My main concern is however changes of the higher taxon. If a species was originally placed in a genus circumscribed as leaf spots producing elliptical spores on a specific host plant, and is moved into a genus with defined conidiogenesis, we have changed the circumscription, and previous "identifications" may now be misapplied names.
The central point is that in practice, significant experience and knowledge about past practices is required, to determine whether a new taxon ID should be given or not. Computer typists will get it wrong and experts may disagree about this.
My conclusion is that if every change in genus name is given a new ID, the system is manageable after assignment of ID. Computers can synonymize IDs (sameAs), but they cannot retrospectively split them. :-)
Gregor