I converted a postcode field to a drop-down field in the checkout. This code is working fine but at the moment I need to show the already stored value of postcode field as a selected option in the checkout fields.
$fields['billing']['billing_postcode'] = array(
'type' => 'select',
'label' => __('Locality - Postcode', 'woocommerce'),
'placeholder' => _x('Select a locality', 'placeholder', 'woocommerce'),
'required' => true,
'class' => array('form-row-first'),
'clear' => false