Change the footer message field in HTML format ?

Hi,

I looking for change the footer message field in HTML format but I tried to modify this :

in advpoll.install - line 110

'footer_message' => array(
        'description' => t('Optional text to display in the footer of the poll.'),
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
)

by

'footer_message' => array(
        'description' => t('Optional text to display in the footer of the poll.'),
        'type' => 'has_body',
        'not null' => FALSE,
        'default' => '',
)

and in advpoll.module - line 428

$form['footer_message'] = array(
        '#type' => 'textfield',
        '#title' => t('Footer Message'),
        '#description' => t('Optional message that can appear below the poll.'),
        '#default_value' => $node->footer_message,
        '#maxlength' => 255
    );

by

$form['footer_message'] = array(
	'#type' => 'has_body',
        '#title' => t('Footer Message'),
        '#description' => t('Optional message that can appear below the poll.'),
        '#default_value' => $node->footer_message
    );

But after desinstall and reinstall the module, in the poll form, the input format well-display but not the footer message field! I don't understand the reason why.

An idea ?
Thanks you very much