Replace required asterisk with text

See http://codex.wordpress.org/Function_Reference/comment_form

http://www.1stwebdesigner.com/wordpress/comment-form-customization/
May be helpful to you if the following isn’t.

<?php $defaults = array( 'fields' => apply_filters( 'comment_form_default_fields', array(
    'author' => '<p class="comment-form-author">' .
                '<label for="author">' . __( 'Name' ) . '</label> ' .
                ( $req ? '<span class="required">*</span>' : '' ) .
                '<input id="author" name="author" type="text" value="' .
                esc_attr( $commenter['comment_author'] ) . '" size="30" tabindex="1"' . $aria_req . ' />' .
                '</p><!-- #form-section-author .form-section -->',
    'email'  => '<p class="comment-form-email">' .
                '<label for="email">' . __( 'Email' ) . '</label> ' .
                ( $req ? '<span class="required">*</span>' : '' ) .
                '<input id="email" name="email" type="text" value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30" tabindex="2"' . $aria_req . ' />' .
                '</p><!-- #form-section-email .form-section -->',
    'url'    => '
<p class="comment-form-url">' .</p>

                '<label for="url">' . __( 'Website' ) . '</label>' .
                '<input id="url" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" tabindex="3" />' .
                '

<!-- #<span class="hiddenSpellError" pre="">form-section-url</span> .form-section -->' ) ),
    'comment_field' => '<p class="comment-form-comment">' .
                '<label for="comment">' . __( 'Comment' ) . '</label>' .
                '<textarea id="comment" name="comment" cols="45" rows="8" tabindex="4" aria-required="true"></textarea>' .
                '</p><!-- #form-section-comment .form-section -->',
    'must_log_in' => '
<p class="must-log-in">' .  sprintf( __( 'You must be <a href="%s">logged in</a> to post a comment.' ), wp_login_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ) . '</p>

',
    'logged_in_as' => '
<p class="logged-in-as">' . sprintf( __( 'Logged in as <a href="%s">%s</a>. <a title="Log out of this account" href="%s">Log out?</a></p>

' ), admin_url( 'profile.php' ), $user_identity, wp_logout_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ),
    'comment_notes_before' => '<p class="comment-notes">' . __( 'Your email is <em>never</em> published nor shared.' ) . ( $req ? __( ' Required fields are marked <span class="required">*</span>' ) : '' ) . '</p>',
    'comment_notes_after' => '<dl class="form-allowed-tags"><dt>' . __( 'You may use these <abbr title="HyperText Markup Language">HTML</abbr> tags and attributes:' ) . '</dt> <dd><code>' . allowed_tags() . '</code></dd>',
    'id_form' => 'commentform',
    'id_submit' => 'submit',
    'title_reply' => __( 'Leave a Reply' ),
    'title_reply_to' => __( 'Leave a Reply to %s' ),
    'cancel_reply_link' => __( 'Cancel reply' ),
    'label_submit' => __( 'Post Comment' ),
); ?>

those are defaults, here’s modified:

<?php $defaults = array( 'fields' => apply_filters( 'comment_form_default_fields', array(
    'author' => '<p class="comment-form-author">' .
                '<label for="author">' . __( 'Name' ) . '</label> ' .
                ( $req ? '<span class="required">Required</span>' : '' ) .
                '<input id="author" name="author" type="text" value="' .
                esc_attr( $commenter['comment_author'] ) . '" size="30" tabindex="1"' . $aria_req . ' />' .
                '</p><!-- #form-section-author .form-section -->',
    'email'  => '<p class="comment-form-email">' .
                '<label for="email">' . __( 'Email' ) . '</label> ' .
                ( $req ? '<span class="required">Required</span>' : '' ) .
                '<input id="email" name="email" type="text" value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30" tabindex="2"' . $aria_req . ' />' .
                '</p><!-- #form-section-email .form-section -->',
    'url'    => '
<p class="comment-form-url">' .</p>

                '<label for="url">' . __( 'Website' ) . '</label>' .
                '<input id="url" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" tabindex="3" />' .
                '

<!-- #<span class="hiddenSpellError" pre="">form-section-url</span> .form-section -->' ) ),
    'comment_field' => '<p class="comment-form-comment">' .
                '<label for="comment">' . __( 'Comment' ) . '</label>' .
                '<textarea id="comment" name="comment" cols="45" rows="8" tabindex="4" aria-required="true"></textarea>' .
                '</p><!-- #form-section-comment .form-section -->',
    'must_log_in' => '
<p class="must-log-in">' .  sprintf( __( 'You must be <a href="%s">logged in</a> to post a comment.' ), wp_login_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ) . '</p>

',
    'logged_in_as' => '
<p class="logged-in-as">' . sprintf( __( 'Logged in as <a href="%s">%s</a>. <a title="Log out of this account" href="%s">Log out?</a></p>

' ), admin_url( 'profile.php' ), $user_identity, wp_logout_url( apply_filters( 'the_permalink', get_permalink( $post_id ) ) ) ),
    'comment_notes_before' => '<p class="comment-notes">' . __( 'Your email is <em>never</em> published nor shared.' ) . ( $req ? __( ' Required fields are marked <span class="required">Required</span>' ) : '' ) . '</p>',
    'comment_notes_after' => '<dl class="form-allowed-tags"><dt>' . __( 'You may use these <abbr title="HyperText Markup Language">HTML</abbr> tags and attributes:' ) . '</dt> <dd><code>' . allowed_tags() . '</code></dd>',
    'id_form' => 'commentform',
    'id_submit' => 'submit',
    'title_reply' => __( 'Leave a Reply' ),
    'title_reply_to' => __( 'Leave a Reply to %s' ),
    'cancel_reply_link' => __( 'Cancel reply' ),
    'label_submit' => __( 'Post Comment' ),
); ?>

Maybe this is what you’re looking for?

<?php
function comment_form_fixer($content){
return str_replace('*', ' (Required)', $content);
}
add_filter('comment_form', 'comment_form_fixer');
?>

Tested this one, works for sure:

function modify_fields_required_marker($fields) {
$commenter = wp_get_current_commenter();
$req = get_option( 'require_name_email' );
$aria_req = ( $req ? " aria-required='true'" : '' );
$fields['author'] = '<p class="comment-form-author">' . '<label for="author">' . __( 'Name' ) . '</label> ' . ( $req ? '<span class="required"> (Required)</span>' : '' ) .
		         '<input id="author" name="author" type="text" value="' . esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . ' /></p>';
$fields['email'] = '<p class="comment-form-email"><label for="email">' . __( 'Email' ) . '</label> ' . ( $req ? '<span class="required"> (Required)</span>' : '' ) .
		         '<input id="email" name="email" type="text" value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></p>';
$fields['url'] = '<p class="comment-form-url"><label for="url">' . __( 'Website' ) . '</label>' .
		         '<input id="url" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" /></p>';
return $fields;
}
add_filter('comment_form_default_fields','modify_fields_required_marker');

I tested, I thought that should work but it still doesn’t...