Comment 7 for bug 871893

I experienced the same thing and freaked when I couldn't access the databases. Same thing, 11.04 upgrade to 11.10. Postgres 8.4 and postgres 9.1 got installed side-by-side.

However, if you check the config at /etc/postgresql/9.1/main/postgresql.conf, it listens on port 5433 and postgres8.4 would be on 5432.

Of course, you have to dump/restore the data from the 8.4 instance to the 9.1 instance. Once you verify that the data is fully transferred, you can blow away the 8.4 instance, and update its config to use port 5432 as normal.