Before you can update your site to Pro you'll first need to convert your X License into an Pro License. As long as you registered your X license prior to the launch of Pro, you can convert your license at no additional charge.
Converting Your License
First of all head over to your account page which you'll find here. Or click on your account in the left hand sidebar, then click Licenses
Once you've clicked that, you should see the manage licenses screen like this:
If you're eligible for a free conversion you should see the Go Pro Free label. Click on that:
Once you've clicked that you'll be redirected to the checkout to upgrade at no charge. You should see this screen:
Click on Convert Now you'll then see a modal like this:
This is to confirm that you want to convert your X license into an Pro license. Licenses can not be converted back. If you're happy with everything go ahead and click on Yes, proceed.
After you've done that, you'll be redirected back to your account where you should see the Pro license and date that you converted the license like this:
Before You Update to Pro
As with any upgrade you should first test the process on a staging/development environment and not your live site. If you aren't upgrading from X 4.6.4, you should first update to the latest version of X and then proceed to update to Pro. The upgrade to Pro has not been tested from X 3.X.X. In addition, make sure you've taken a backup of your site before proceeding with any upgrades to ensure you have a version you can roll back to.
Upgrading to Pro from X couldn't be simpler. Simply upload Pro either via FTP or via your wp-admin > Appearance > Themes > Install and activate. All your settings and content will be automatically migrated to Pro from X.
You may notice that when you activate Pro, it automatically deactivates Cornerstone. This is because Pro uses a native family of builders of which the Cornerstone you know currently is just a small part.If you are using Pro you will no longer need to use Cornerstone as a separate plugin. If you are using X you will continue to use Cornerstone as you always have as a separate plugin.
Ultimately the choice is up to you as to which product you wish to use. Both X and Pro will continue to be updated and supported with the main difference between the two being a family of interconnected builders that are included in Pro.
You are welcome to keep using X for all your licenses, convert some of your licenses to Pro, or convert all of your licenses to Pro. Going forward, you have the choice of X or Pro based on the needs of your project. One product is not lesser or greater than the other.
Updating Your Child Theme
If you are currently using a custom child theme, we'd strongly recommend testing any updates to Pro on a staging area beforehand while your custom code should work keep in mind the header and footer builder introduces new CSS classes and markup which means you may need to modify your child theme. In addition, you'll need to change the template name in your child theme to xpro. While you may wish to change the folder name, you only technically need to change the template name. This means the top of your style.css in your child theme should look something like this:
Theme Name: Pro – Child Theme
Theme URI: http://theme.co/x/
Author URI: http://theme.co/
Description: Make all of your modifications to Pro in this child theme.
You've now successfully converted and updated your site to Pro.