extending or modified use of an ontology?

I didn't get satisfactory responses to my earlier email on FOAF, so I'm repeating a query below, which I guess can be applied to other ontologies as well:

What is the way to modify/adapt (subtle or not not subtle) the semantics of a term / property in an ontology-- example I don't want foaf:interest to be a "document", as in, foaf:Document. I may want it to be just a list of "terms" or concepts, or maybe "interest" in my application domain is not a document, but much broader, say owl:Thing, or whatever...  Do I define my own property, or inherit from the foaf property? How do I "extend" foaf:interest ? In general what are the semantics of "is-a"; are they as strict as in OO type inheritance, where EVERYTHING TRUE about the super class must also atleast hold true about the subclass? I've seen IS-A used "loosely" in several RDFS/OWL documents. In FOAF for instance, many wide "wordnet concepts" are said to parents of foaf properties, but those wordnet concepts are open to wide interpretation, fuzzy semantics, and I'm not sure how we can definitively say, my class IS-A particular wordnet concept. What's an
 organizing principle in situations such as these?



Regards,
Revi