Pre-Populating a Form Field with something other than its default value
Figured it out on my own. The solution was:

$form['restaurant_newsletter_subscription'] = array(
    '#type' =&gt; 'checkbox',
    '#title' =&gt; t('Subscribe to Restaurant Newsletter'),
    '#return_value' =&gt; "True",
    '#default_value' =&gt; $attributes[15]-&gt;Value,
    '#prefix' =&gt; '&lt;div class="subscription-toggle"&gt;',
    '#suffix' =&gt; '&lt;/div&gt;',
    '#ajax' =&gt; array(
        'callback' =&gt; 'ajax_replace_default_callback',
        'wrapper' =&gt; 'restaurant_newsletter_subscription_div',
    ),
);


And for the callback:

function ajax_replace_default($form, $form_state) {
    $field_name = $form_state['triggering_element']['#name'];
    $form[$field_name]['#default_value'] = "False";
    return $form[$field_name];
}


And I also kept the hidden fields in the form:

$form['existing_restaurant_newsletter_subscription'] = array(
    '#type' =&gt; 'checkbox',
    '#title' =&gt; t('Restaurant Newsletter'),
    '#value' =&gt; $attributes[15]-&gt;Value,
    '#access' =&gt; FALSE,
);


This was simpler to store the existing preference variables for comparison in my logic later on than to try to retrieve the subscriber yet again inside of that form submit handler (It already retrieved the subscriber when it loaded the form, to populate the fields....trying to minimize the number of API calls I have to make).

Implemented like this, if a box is checked and becomes unchecked, that Ajax callback function alters the default value to false, rather than returning just the pre-populated value. If you check the box again it still changes the default to false, but it doesn't matter since if the box is checked that's irrelevant.

Not the most elegant fix, you get a little loading circle thing for a second when the Ajax runs, but it works.

Note: I left the Ajax callback function generic because even though I keep using the same field on this page, there are lots of similar fields doing the same thing. Using the triggering_element allows me to just recycle this code for all of them.