A database upgrade from Oracle9i Release 1 to Oracle 10g Release 2 is a substantial change. Because I have no information about your applications or databases, it is impossible for me to predict whether you will encounter any difficulties as a result of this upgrade.
The upgrade guide, in particular, contains excellent information on how to prepare for an upgrade, as well as perform sufficient functional testing. I would recommend practicing the upgrade several times in a non-production environment. This testing should include application and functional testing each time the upgrade is performed. Be sure to create a test plan prior to testing, and make sure it is followed by everyone testing the supported applications.
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