Find out the steps and actions needed to change the Business Connector Proxy account in Microsoft Dynamics AX after you have EP installed, and SSRS and SSAS configured for EP by reading this post on the AX Support Team’s blog.

http://blogs.msdn.com/b/axsupport/archive/2012/09/13/how-to-change-the-business-connector-account-after-enterprise-portal-is-already-installed.aspx