I believe that you will have to test if you wish to become licensed as PE Controls engineer in addition to being a PE Power licensee. I question why you would wish to take the PE control examination if you already have the PE Power license?
If you are in a discipline license state, then I think that I understand. If you are in a general license state, as I understand the licensing laws and processes, do not believe that you need to retake the PE Controls examination. Of course, you are only ethically allowed to seal in those areas where you have competence, as manifested by either examination, experience or both. However, if you work in the controls area and develop an experiential competence in that area, then, it is my understanding that you would not need to gain a second PE license. Again, if you are in a jurisdiction which licenses by discipline, then I understand what you are going to do.
Congratulations on passing the PE Power! Good Luck on the PE Controls.