As it is Debian based:

pg_dropcluster --stop 8.4 main
pg_upgradecluster 8.3 main
pg_dropcluster --stop 8.3 main
apt-get remove postgresql-8.3

The first command is needed as the 8.4 instance already has a "main" cluster which would block the upgrade. If your cluster is named differently, it is not needed. The upgraded cluster will grab the port of the old one.


Good luck with the flooring upgrade ;-)