It's not very easy to find information on how to adjust debconf settings after a package has been installed and configured. Most of the information out there is for Debian developers wanting to add support for debconf in their maintainer scripts.
I ran into the problem of being unable to change a package's configuration options through dpkg-reconfigure and I found the following commands to do it manually:
to show the list of debconf values that a package has stored,
echo "get packagename/pgsql/app-pass" | debconf-communicate
to query the current value of an option in the debconf database, and
echo "set packagename/pgsql/app-pass password1" | debconf-communicate
to change that value.
I'm not convinced that this is the easiest way for system administrators to manually lookup and modify debconf options, but that's the best I could find at the time.