Hi,
So it’s a yes and no kind of answer.
If you wish to be absolutely certain of things, you should check if your host offers what’s known as a “staging” environment, this is basically a copy of your website where you can do any kind of testing you would like based on how the site looked when you started.
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Otherwise, enabling Gutenberg will not change any existing content, you can then create a new post or page to play around with and just preview it instead of publishing it for everyone to see.
If you then want to test on existing content, open any single piece of content, it will show up in a Classic block (this is the classic editor view you’re already familiar with, inside Gutenberg), clicking the ellipsis on the side of that block will allow you to convert it into blocks and you can then see if that works for you. Should it not work, just open it in the classic editor and use the revisions of it to go back to before it was made into blocks.
The last bit does require you to have revisions enabled on your site, which not everyone has (you’ll see it says how many revisions you have when you edit a post or page if it’s enabled), and that you do not save so many times so that your revision gets overwritten (as there’s a limit to how many revisions you will have for each post or page).
