When you build an update package with B733.2 and up (don't know about
previous versions), the parent package that you build the update upon is
updated automatically. You can verify this by looking at the deployment
server under the update package name, and checking some of the object
dates/times, such as in the spec directory. Then go to the parent package
the update was built upon, and those objects should now have the new
date/time as reflected from the update package directory.
Please allow me to point out that you will still need to compress the specs (at least) for the full package b4 the full package is truly updated. The update package build updates, but does not compress, the full package.