Full website rebuild with 500 pages changing. Best way to protect SEO?

Need some honest SEO advice before I relaunch my site. My current website has been live for around 6 years. I am now rebuilding the whole thing with a new design, new messaging, and new content sections like tools, white papers, and other resources. The part I am worried about is this: There are about 500 existing pages on the current site, and a lot of them are going to change significantly. Some will be rewritten, some replaced, some merge