Is it possible to change the Opt in checkbox at the checkout page for woocommerce?

Hi @geomeach - I’m not super familiar with WooCommerce’s code base or your GDPR compliance in UK, but throwing out an idea to consider.  Since you’re on WooCommerce and you can edit the code/template of the Checkout page, I would recommend just having the checkbox auto-selected instead of inverting its functionality. 

As am example, you can look for the input checkbox in the HTML and set it to checked like this:

<input type="checkbox" class="input-checkbox " name="kl_newsletter_checkbox" id="kl_newsletter_checkbox" value="1" checked="true" />

If that doesn’t work, you can use Javascript to do this after the page renders:

<script type="text/javascript">
window.onload = function() {
   document.getElementById('kl_newsletter_checkbox').checked = true
}
</script>

I haven’t tested the Javascript code above, so you may have to adjust it, find the proper placement, or add it to your existing Javascript functions.  But it should give you or your developer a head start!