Drupal AJAX Replace - ajax

Question

I'm creating a custom user settings page. I have one field: zip_code that get's it's initial value from a custom user field. I have a custom function that pulls external data using the value of the zip_code.
I currently have the default value of the field set to the custom user field (if it's available). This is working as designed; however, I want to give the user the ability to change their zip code via an Ajax callback. This would replace the already populated radio buttons with new ones. I can't seem to wrap my head around this. Here's my code:
function settings_shopping_form($form, &$form_state) {
include_once "external.inc";
// Get user fields
global $user;
$user_fields = user_load($user->uid);
$zipcode = $user_fields->zip_code['und']['0']['value'];
if(isset($zipcode)) {
$form['zip_code'] = array(
'#title' => t('Zip Code'),
'#type' => 'textfield',
'#required' => TRUE,
'#default_value' => $zipcode,
'#ajax' => array(
'callback' => 'settings_form_callback',
'wrapper' => 'textfields',
),
);
$storename = getmystorename($zipcode);
if(count($storename) > 0) {
$form['textfields'] = array(
'#prefix' => '<div id="textfields">',
'#suffix' => '</div>',
'#type' => 'fieldset' );
$form['textfields']['stores'] = array(
'#type' => 'radios',
'#title' => t('Choose your store:'),
'#options' => $storename,
'#default_value' => $storename[1], );
} else {
$form['textfields']['incorrect'] = array(
'#title' => t('Sorry, there are no stores available near you. Check back later'),
'#type' => 'fieldset', );
}
}
My callback function is very simple:
function settings_form_callback($form, $form_state) {
return $form['textfields'];
}
To reiterate: I want to add the ability to replace the populated radio buttons with new buttons generated by the getmystorename function when the zip_code field is changed.

Answer 1

I ended up taking an example from the examples module (love it!):
$defaults = !empty($form_state['values']['zip_code']) ? $form_state['values']['zip_code'] : $zipcode;
$storename = getmystorename($defaults);
I put this before the start of my form so that the values load before the form builder.