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

The simple answer is no.

When you install the PRO 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. 

For Zerif Pro there are edge cases after activating where you might have to re-add the images for the widgets and remove the team widget in the footer by going to Appearance->Widgets and ensure that there is no team widget in the footer.

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 if you make a back up. You can simply do a back up with a plugin, and for that we suggest  Duplicator or Customizer Export/Import plugins.

Uploading the PRO theme

Go to Appearance-> Themes-> Add New and try to upload the theme.

After you install the PRO theme, you can start by clicking Preview instead of Activate, in order to see if your website transferred well.