Commerce - How to hook into ajax call on commerce cart form variation change
There's a hook for that: hook_commerce_cart_attributes_refresh_alter. This is invoked when the variation is selected by the user and reacts to changes in the cart widget.

It gives you the chance of using ajax commands, so you basically have the complete control over elements and behaviors, there's an excellent tutorial in here: https://drupal.stackexchange.com/a/73325/2322

Also, a couple of examples, Commerce itself invokes the hook from the cart module
and there's this sandbox module called Commerce Single Page Checkout that uses the technique for adding a view on the refresh event using ajax_command_replace.