Taken from:

[url]http://support.oracle.com.sg/metalink/plsql/ml2_documents.showFrameDocument?p_database_id=NOT&p_id=117048.1[/url]

The following steps need to be taken to convert your SE database to an EE.

1. Backup the database
2. De-install the SE software
3. Install the EE software
4. 'Create' a new database (for expample on NT. create a new service)
5. Startup the database
6. run catalog and catproc.

There is also a note about it for 8.1.5 here:
[url]http://support.oracle.com.sg/metalink/plsql/ml2_documents.showFrameDocument?p_database_id=NOT&p_id=101317.1[/url]