Do something if product type is changed

WooCommerce set the product type with JavaScript and is not set wit PHP. They use the change function if the page is loading and set the right product type on screen.

If you need a function on a specific product type you can do something like this

$('#product-type').on('change', function() {
    if($('#product-type').val() == '//your type'){
        //do something
    }
});

Put your code inside the ready event. The code inside this function fires only when the DOM is completely loaded.

$(document).ready(function(){
    $('#product-type').on('change', function() {
        //do something
    });
});