How do I change the content text programmatically?
This value is defined in the comment_form function. More specifically, it is the description in this section:

  // Add author e-mail and homepage fields depending on the current user.
  $form['author']['mail'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('E-mail'),
    '#default_value' =&gt; $comment-&gt;mail,
    '#required' =&gt; (!$user-&gt;uid &amp;&amp; $anonymous_contact == COMMENT_ANONYMOUS_MUST_CONTACT),
    '#maxlength' =&gt; 64,
    '#size' =&gt; 30,
    '#description' =&gt; t('The content of this field is kept private and will not be shown publicly.'),
    '#access' =&gt; $is_admin || (!$user-&gt;uid &amp;&amp; $anonymous_contact != COMMENT_ANONYMOUS_MAYNOT_CONTACT),
  );


You could create a custom module and implement hook_form_FORM_ID_alter. Your custom module file would contain below lines in a .module file and a basic .info file.

&lt;?php

function MODULENAME_form_comment_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['author']['mail']['#description'] = t('E-mail address will not be shown publicly.');
}


Alternatively there are some options for overriding strings outlined here: Drupal 6 &amp; 7 - Change default strings (text) without using full translation system