Which method or action hook is fired when changing shipping method on checkout page?

I need to change the address fields to be 'required', when the user will select shipping method that is other than "local pickup".

I managed make it optional by default (local pickup is selected by default) using a filter, like this:

add_filter( 'woocommerce_checkout_fields', 'disable_required_address_shipping' );

function disable_required_address_shipping($fields ) {
    if ( check_if_local_pickup() ) {

        $fields['billing']['billing_address_1']['required'] = false;
        $fields['billing']['billing_address_2']['required'] = false;
        $fields['billing']['billing_city']['required'] = false;
    }

    return $fields;

}

function check_if_local_pickup() {
    $chosen_methods = WC()->session->get( 'chosen_shipping_methods' );
    $chosen_shipping = $chosen_methods[0]; 
    if ($chosen_shipping == 'local_pickup') { 
        return true;
    }
}

But I need to change it dynamically using an action.

I tried to look for a solution here and couldn't find one.

Is there any action that fires when changing shipping method?
Any ideas?

via Chebli Mohamed