Davit

/****** Customize admin message "Howdy" to "Welcome" ******/
$nohowdy = "Hello!";

if (is_admin()) {
    add_action('init', 'artdev_nohowdy_h');
    add_action('admin_footer', 'artdev_nohowdy_f');
}
// Load jQuery
function artdev_nohowdy_h() {
    wp_enqueue_script('jquery');
}
// Modify
function artdev_nohowdy_f() {
global $nohowdy;
echo <<<JS
<script type="text/javascript">
//<![CDATA[
var nohowdy = "$nohowdy";
jQuery('#user_info p')
    .html(
    jQuery('#user_info p')
        .html()
        .replace(/Howdy/,nohowdy)
    );
//]]>
JS;
}

add_filter('gettext', 'change_howdy', 10, 3);

function change_howdy($translated, $text, $domain) {

    if (!is_admin() || 'default' != $domain)
        return $translated;

    if (false !== strpos($translated, 'Howdy'))
        return str_replace('Howdy', 'Hello', $translated);

    return $translated;
}