Oracle 11g Enterprise downgrade to Standard One
Ultimate source for such answers is Oracle Database Licensing Information.

Sadly to downgrade from Enterprise to Standard Edition you have to export all the data install Oracle Standard Edition and import data (Doc ID 139642.1).

From your list "Automatic SQL Tuning Advisor" is Enterprise Edition only feature. As a rule of thumb - features which needs AWR are Enterprise Edition only plus you need to buy Oracle Diagnostics Pack.