> However, do you know if a server might respond 
> that 'no change has happened' even though a file was 
> changed? If so, this could be a big problem (not 
> updating changed files on an update). So, do you 
know 
> of anything that could be done using HTTrack to very 
> reliably get changed files (besides downloading 
> everything each time)?
No, this (should..) never happends. You can uncheck 
the "update hack" option, however : this option forces 
the "not updated" flag on certain cases (especially 
when the server gives a supposedly updated page, but 
which has the same size than the old one)

> I would be willing to spend some time on this, 
> including changing the code if need be, even adding 
it 
> as a command-line option and sending the changes to 
> you for your approval. Just wondering if any you 
have 
> any hints.

Again, the only problem encountered if the contrary: 
unnecessary updated generated by dynamic contents

> many thanks for a truly great piece of software, 
> and even more thanks for providing the source,
> john

You're welcome :)