There are a couple of possibilities for customizing this.

There are two filters – wpmem_register_links_args and wpmem_register_links that allow you to filter the links that display for the logged in state of the register form shortcode.

Or, you can create a custom logged in state by using content nested in the shortcode (i.e. [wpmem_form register]This is my logged in content.[/wpmem_form]). See: http://rocketgeek.com/plugins/wp-members/docs/shortcodes/form/