Please edit your X child theme's functions.php and copy its entire content, then with that content, replace the Pro child theme's functions.php content. Then, copy the rest of the file from your X child theme to Po child theme (except style.css). Style.css is what defines your child theme so you can't just copy it. Instead, if you have existing custom CSS in X Child theme's style.css, just copy it to your Pro Child theme's style.css (not the entire content, just your added custom CSS).
This process is only good if your existing X child has the latest structure and custom code, but if it's from old one then you'll have to do a manual coding to make it compatible to the latest Pro. And you must contact a developer to do itr. This is the same as site maintenance as you'll have to fix and update your custom code, and you should do this in a staging site (not your live site).