Ontology evolution

To the W3C Ontology Working Group -

In section 3.2., Ontology evolution, of Requirements for a Web Ontology
Language, you state

    "An important issue of revision is whether or not documents that commit
to one version of an ontology are compatible
     with those that commit to another. Both compatible and incompatible
revisions should be allowed, but it should be
    possible to distinguish between the two. Note that it is possible for a
revision to change the intended meaning of a term
    without changing its formal description.. Thus determining semantic
backwards-compatibility requires more than a
    simple comparison of term descriptions. As such, the ontology author
needs to be able to indicate such changes
    explicitly. "

I don't get it. How can the meaning of a term within a universe of discourse
change, yet its formal description remain the same? Isn't that what
ontologies are for - to encode meaning? Unless you intend that "formal
description" simply refers to the human readable documentation comments
within the ontology, and not the encoded semantics of the term. If so, that
is not clear from the above. Please clarify.

Thank you,

Hal Noyes
Oracle DBA
Howard Systems International