The migration guide describes a number of different methods for migrating to Oracle9i. Keep in mind that a solid...
backout plan is very important in case the migration does not go well in the production environment.
Second, I would recommend testing this migration several times in a non-production environment; be sure to involve the application support team members, as well as business users. Each time the migration is completed in the test environment, everyone involved should test application functionality, batch jobs and other critical functions thoroughly.
With respect to your question about using export/import to perform the migration and then finding a delta, export/import essentially dumps your data and object definitions into a binary file which you then use to import into another database. It basically provides a 'snapshot' of your database (or schema or object) at the time the export is performed. There is really no way to then perform another export with only data that has changed since the last export. For more information on Oracle export and import utilities, you can review the Oracle9i Release 2 (9.2) Database Utilities guide.
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