Will this break my site?
No. Your site will not automatically update to the new editor at the date of release, it must be manually upgraded by an Administrator. Until that happens, no changes will be present in your site. Once WordPress 5.0 has been installed, you will see messages that the new editor is in place. You will find it when you edit any page or post. At this time, we cannot guarantee that your current theme will account for the new layout created by Gutenberg. For example, you may find that the new block styles need adjustment within your theme to ensure that layouts benefit from the power of the new editor – otherwise, your posts will fall back to using global styles.
Existing themes may have been configured to use Advanced Custom Fields (ACF), which is a professional plugin that WordPress developers have used to extend posts to have custom fields with robust configuration options. ACF is not going anywhere – it still serves a purpose in creating complex layouts, lists of data from other areas of the site, and to create relationships between content. It also comes with a mature User Interface and templating language. We will continue to use ACF for non-standard content types.
How will this change my workflow?
Where we will see Gutenberg shine is in creating blog posts, rich media resources, marketing landing pages and other types of content that doesn’t need heavy design or page layout – but rather focus on publishing quality content in an attractive layout with predictable elements with no coding required.
Instead of a single WYSIWYG editor for all post content, posts are now comprised of ‘blocks’. Picture this: You are an editor working on a new blog post. You create a new post, fill in the title, and click a + button to select your blocks. First, you chose a cover block, which adds an image to fill the top of the page, next you chose a subtitle, fill in some paragraphs, add a pull quote to highlight important content, add a formatted block of images, and round it out with your remaining copy blocks.
And for further peace of mind, according to the WordPress team, the classic editor will be supported into 2021.