Your old content will not disappear! However, the experience is a little bit different. The block editor will put all the old content into a single Classic Block. The Classic block is basically the old TinyMCE editor, but embedded inside the new editor.
To work with your old content you can:
- Leave it as a Classic block and edit it like in the old editor
- Use the Three dots icon and select Convert to Blocks. This will break it into individual blocks and you'll be able to edit it with the new block editor.
Once you're finished, click the Publish button.