Will I lose my customizations if I upgrade to the Pro/Plus version of the theme?

The simple answer is no.

When you install the PRO/PLUS version of the theme, after you already had the free one, all the content from the previous theme should transfer automatically without issues.

A thing you should check before you upgrade is if you have the latest version of the free theme. Also, if you've made changes done to the theme files directly, without using a child theme or a plugin, those will be lost. 

Back up your theme files

You can also make a back up of your current website. Because you are upgrading the theme files, there is a chance something could go wrong so it’s best to play it safe and have a backup. You can simply do a back up with a plugin, and for that we suggest   Duplicator or Customizer Export/Import plugins.