If you are using a Standard Edition database (Release prior to 11gR1), then you can change it to an Enterprise Edition database.

Step 1
Standard Edition database software should be same as the Enterprise Edition database software.
Step 2
Shutdown the database
Step 3
Shut down your all oracle services including oracle database
Step 4
De-install the Standard Edition oracle software
Step 5
Install the Enterprise Edition server software using the Oracle Universal Installer.
Step 6
Select the same Oracle home that was used for the de-installed Standard Edition. During the installation, be sure to select the Enterprise Edition. When prompted, choose Software only from the Database Configuration screen.
Step 7
Start up your database.
Your database is now upgraded to the Enterprise Edition.

Tips:

1. You can only convert Standard Edition Database to the Enterprise Edition Database by using above method.

2. If you want to convert from an Enterprise Edition database to a Standard Edition database, you must use Export/Import operation. Without Export/Import you can not convert.


Inside Story:

1. The Enterprise Edition contains data dictionary objects which are not available in the Standard Edition. If you just install the Standard Edition software, then you will end up with data dictionary objects which are useless. Some of them might be invalid and possibly create problems when maintaining the database.

2. The Export/Import operation does not introduce data dictionary objects specific to the Enterprise Edition, because the SYS schema objects are not exported. Oracle recommends using the Standard Edition EXP utility to export the data.

3. After the Import in the Standard Edition database, you are only required to drop all user schemas related to Enterprise Edition features, such as the MDSYS account used with Oracle Spatial.