How to use owl:versionInfo for Classes?

First of all, the "rdf:ID vs rdf:about" question doesn't take you any further here, since it is only about a certain aspect of the RDF/XML syntax and has nothing to do with RDF or OWL itself. (Just as a side note, using rdf:about always is probably a good idea, as long as you write your RDF/XML by hand [which is, however, probably never a good idea :-)] ).

Your real question appears more to be a question of ontology management, more concretely: ontology update. It should be clear that you should not simply add the new definition of class A to the ontology, because then you would have the axioms from both the old and the new definition for A in your ontology - bad idea! Rather, you have to replace the old class definition. (And in this case, there is not even a need for discussing the "ID vs about" question then, just as another side note.) So, actually, you are about to create a new version of your original ontology, which then contains the new definition of the class A. And this brings us to the topic of ontology versioning!

There is some discussion about ontology versioning in the OWL 2 spec:

http://www.w3.org/TR/2009/REC-owl2-syntax-20091027/#Versioning_of_OWL_2_Ontologies

The basic idea is to always have two URIs for your ontology:

• a generic ontology URI, which always refers to the most-current version of the ontology;
• a specific version URI, which refers to a concrete version and sticks with it.

The ontology URI is just the URI of the ontology header, as it has always been. The version URI is defined by property owl:versionIRI.

So, in your case, the situation might look as follows (using Turtle instead of RDF/XML for good reasons :-)):

Old version of ontology:

<http://example.org/myontology/> 
    rdf:type owl:Ontology ;
    owl:versionIRI <http://example.org/myontology/v1/> .
... old definition of class A ...

New version of ontology:

<http://example.org/myontology/> 
    rdf:type owl:Ontology ;
    owl:versionIRI <http://example.org/myontology/v2/> ;
    owl:priorVersion <http://example.org/myontology/v1/> .
... new definition of class A ...

If you like, you might add an additional owl:incompatibleWith statement to the new version of the ontology, with the version URI of the old version in object position.

In both versions of the ontology, the URI prefix of all defined entities, including class A, should be that of the ontology URI, not the version URI. Otherwise, it would not make much sense anymore to talk about different versions of the "same" ontology.

Edit: In order to keep track of the version of the class A itself, it is still a good idea to add the property owl:versionInfo to the class in any of the ontology versions. So you would have:

In old version of ontology:

:A rdf:type owl:Class ;
    owl:versionInfo "v1" ;
    ... old definition of class ...

In new version of ontology:

:A rdf:type owl:Class ;
    owl:versionInfo "v2" ;
    ... new definition of class ...

I know a website which has detailed owl web, you can go in and have a look http://www.w3.org/TR/owl-guide/.