First of all let’s dispel some myths or worries (if there are any left at this point). It will indeed come with a grace period and be backwards compatible with “old school” Word Press.
The Gutenberg FAQ is a great place to start reading about how this will work and the commitments the Gutenberg team has made in regards to it.
This step is essential to automatically update the required plugins for bug fix and feature improvement.
If you have not done customizations to the theme, or you are running a child theme you can: If you have made modifications to your theme files (for example your custom.css), you need to reapply them to the new version.
We recommend using a child theme to avoid this step.
Step 2: Copy the custom code you made in your site and save it in your computer to add it later.
To help you easier with your custom files, the Child Theme is suggested to be installed and all the customization should be done there.