Conversion from 8.4 to >9 version question

Hope this has not been just recently covered if so apologies.

I'm after a simple way to upgrade from 8.4 to the latest version of
postgres (my server is a ubuntu 10 LTS I think)

When I back up my work database I just use pgadmin > backup as plain
text > take it home, rebuild it on my home machine which does run 9.1 -
on the command line using: psql databasename -f the-backup-filename
This seems to work ok and my software runs ok and I can access all my data

Can I just do a server upgrade to the latest ubuntu version and postgres
version and then do a similar command line import safely or is this too
simplistic a way of looking at it? If one does do the upgrade what
happens to the actual database itself if one doesn't explicity do anything?

Thanks in anticipatation.

Richard