I devised an interesting way to do this all in RoboHelp.
Basically it involves creating See Also keywords and being diligent
about linking the New or Revised topics to the appropriate
keywords. I then created a "What's New" page that I never really
had to maintain after it was created. All I had to do was ensure I
linked the topics to the appropriate keywords. The What's New page
contained See Also controls that pointed to the keywords.
Shoot me a private message rstone75 (at) kc (dot) rr (dot)
com and I'll be happy to send along an example.