November 2012
While migrating from Debian Squeeze to Debian Wheezy...
sudo apt-get install postgresql-9.1
(It's not serving anything yet.)
sudo -u postgres pg_ctlcluster 9.1 main stop
sudo -u postgres pg_dropcluster 9.1 main
NB the order of the versions in the command:
sudo -u postgres pg_upgradecluster -v 9.1 8.4 main
sudo -u postgres pg_dropcluster 8.4 main
sudo apt-get remove -s postgresql-8.4