data format problem for upgrede from 7.1->7.3

Hi:

I just upgraded from 7.1->7.3, but did not take a data back up.
Now when i try to start the posrGreSQL server I get the message:

An old version of the database format was found.\nYou need to upgrade
the data format before using PostgreSQL.\nSee (Your System's
documentation directory)/postgresql-7.3/README.rpm-dist for more
information.

There is not much info on format change in the Docs.

Any idea what might be the resolution here?

Thanks,

Gautam