How to customize Orchard's default FormsAuthentication to my own custom authentication

Question

I need to customize Orchard's default FormsAuthentication to my own custom authentication

Currently FormsAuthenticationService class is checking

if (httpContext.IsBackgroundContext() || !httpContext.Request.IsAuthenticated || !(httpContext.User.Identity is FormsIdentity))
        {
            return null;
        }

and httpContext.User.Identity is returning as System.Web.Security.FormsIdentity instead of FormsIdentity i need to check my own identity like Csla.Security.UnauthenticatedIdentity