If a package has a file marked %config in the spec file then the rule is:

   if the config file hadn't been changed (ie matched that from the
   original package installation) then just replace it with the new one

   if it *has* been changed then rename the existing one to <name>.rpmsave
   and put the new one in place.

If the file is marked as config(noreplace) then the logic turns into:

   if the config file hadn't been changed (ie matched that from the
   original package installation) then just replace it with the new one

   if it *has* been changed then install the new one as <name>.rpmnew

Normally config files would be marked %config(noreplace) unless there is a 
change to the config-syntax, but the package specs sometimes get it wrong, 
especially ones which perhaps date back to before (noreplace) was an 
option (or at least a common one!)

After any update it is usually a good idea to look for .rpmsave/.rpmnew 
files just in case!

Here are some web pages describing this (just picked at random from 
googling)

http://rpm.org/api/4.4.2.2/specfile.html
http://docs.fedoraproject.org/drafts/rpm-guide-en/ch09s05.html#id2972655
http://www-uxsup.csx.cam.ac.uk/~jw35/docs/rpm_config.html

  -- Jon