Building effective CoEs
You might want to start by not calling them centers of excellence. Find a more innocuous name, such as Scrum Advocates, Scrum Implementation Champions, or Scrum Working Groups. That avoids the notion that these folks are the experts who are somehow superior to the Scrum team members. While the CoE staff may have greater knowledge and experiences, they are still learning human beings. If your organization decides to retain the Scrum CoE verbiage, then focus on the word "excellence" as indicating their role to help the organization continuously improve and achieve excellence.
Avoid any notion that the CoEs are somehow in charge of the day-to-day operations of the Scrum teams. While they may have some governance responsibilities, they cannot over-govern. Mostly, they are a body of experienced resources available to the Scrum teams as collaborative and participative partners. Some CoEs have executive powers, but those powers are used to help to remove...