title: Featured image via built-in Wordpress function instead of ACF (Pro) image field
body: <p>I have a news website with over 3000 posts. The previous website builder did not use the built-in functionality of Wordpress for the featured image, but used an ACF (Pro) image field instead.</p>
  <p>On the new website I would like to use the standard function of Wordpress. I just have no idea how to set the images from the ACF (Pro) image field as a featured image via the built-in Wordpress function.</p>
  <p>Is there a script that can do that automatically?
  I don't like having to do more than 3000 messages manually.</p>
  <p>So the image should be from this:</p>
  <p><a href="https://i.stack.imgur.com/LKA5e.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LKA5e.png" alt="enter image description here" /></a></p>
  <p>To this:</p>
  <p><a href="https://i.stack.imgur.com/rLvbA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rLvbA.png" alt="enter image description here" /></a></p>
  <p>I've searched all over the internet for a solution to this, but couldn't find anything.</p>
  <p>Thank you very much in advance for any help.</p>