The schema does include a way to place a timestamp or version on a TaxonConcept? entry. Each of these would be useful for many purposes, and would allow users quickly to determine what may have changed in a set of names or concepts. I would therefore suggest that we include an element for each inside <TaxonConcept?>, e.g. <DateLastModified?> and <Version>.
This is unfortunately not a trivial issue. We need to specify exactly what can be changed without creating a new concept (in which case the AccordingTo would hold the date modified). But you are right, we need to track if spelling errors get corrected etc. But this will only work with a centralised data repository to avoid duplication of version numbers. If these versions are only required for a certain specific purpose, it is possible to add them to the XML with a different namespace and not include the definition in the TaxonSchema?.
I think the simple solution is to renumber for every change. What is considered significant will always vary between different users. There is not likely to be much of a problem with being overgenerous with renumbering.
RobertKukla