A reusable block is meant to be a global block…meaning that you can insert it on any page and if the content is updated, it will update across the entire site. This is handy, for instance, for call-to-action content.
However, if you want to have a reusable block that has all of the same styling and features, but just need to change the text, you can convert an instance of that block back to a regular block by clicking on the three dots (block settings) menu and clicking “Convert to Regular Block”:
