The biggest thing is to install the software in separate ORACLE_HOME directories. Then, use the Database Upgrade Assistant, DBUA, to upgrade your databases from 8i to 9i. Once all databases have been upgraded to 9i, and your listener is running on the 9i ORACLE_HOME, you can then remove your 8i software. I typically keep my older versions of software around for at least a month...just in case....
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