I have been through the process of Upgrading servers/databases from Oracle 8i to 9i. There are a couple of documents I found on Metalink that helped me through.The main thing was first upgrading the servers and then bringing up the database with a 'STARTUP MIGRATE' command. The migrate utility is quite straightforward and it did the rest. I did not have to do any exports/imports. Yes, just to be on the safer side I did a full cold backup of the database and also backed up the ORACLE_HOME directory, in case I needed to back out of the upgrade.
Take a look at Note 159657.1 on Metalink.

All the best.

-Chintz