You can convert DMTs to LMTs using DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL. There are a few limitations relating to space allocation when using this method. It's actually better to create a new tablespace and move the contents across to it.
After the completion of Oracle 9i database creation, can I make the database 'Extent Management Local' and 'define Undo tablespace' later.
There's another option: let the Oracle Database Configuration Assistent create batch files when you design a database. Edit the scripts that are called from the create batch file and alter your 'Extent Management Local' and 'Undo tablespace'-information there. After that run the batch file...
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