In January 2013 the 3rd EU Licence Directive changes were implemented in the UK, which introduced stepped licences and the new categories A1, A2 and A. These changes removed the old route whereby you could take your test before you were 21 and you would be restricted to 33 bhp for 2 years before the licence automatically became unrestricted.
Anyone passing their test under the old rules will continue under those rules, so if you have a restricted 33bhp licence it will automatically become unrestricted after 2 years. You do not need to take another test.