Hello JedJJ,

You don’t need to shut down the A side controller to replace your B side controller.  What you will need to do is to power down SP B.  Once SP B is powered down then you can replace it.  If you have an EMC account, you can go here https://solveonline.emc.com/ and find your system.  Once you have your system selected you can select replacing SP & it will create the step by step guide for you to follow.  

Please let us know if you have any other questions.