How to use 11gR2 deinstall for standard to enterprise upgrade?
...
4. Deinstall the Standard Edition server software.
5. Install the Enterprise Edition server software using the Oracle Universal Installer.
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.
...
When I did this on a 10gR2 setup, I used the "Remove product" option from the OUI but the new installer doesn't have this so you have