function enyc_show_fullname_for_displayed_user( $username ) {
    return bp_get_displayed_user_fullname();
}
add_filter( 'bp_get_displayed_user_username', 'enyc_show_fullname_for_displayed_user' );

function enyc_show_fullname_for_loggedin_user( $username ) {
    return bp_loggedin_user_fullname();
}
add_filter( 'bp_get_loggedin_user_username', 'enyc_show_fullname_for_loggedin_user' );