Especially if you are replacing the SSD, you will need the recovery USB to reinstall ChromeOS on the new SSD, or the Chromebox won't boot.

Follow the instructions from Google's website to create a recovery USB stick: https://support.google.com/chromeosflex/...

You will need an USB stick with at least 8GB of capacity (the contents of the stick will be overwritten!) and a system running the Chrome browser - so it's easiest to do it on the Chromebox before taking it apart for an upgrade.