What happens to our old website's content when we move to a new site?
Audit it, keep what earns traffic, redirect the rest, and do not lose the blog.
Every old page gets one of four fates: keep it, improve it, merge it into a better page, or retire it with a 301 redirect to the closest match. The decision is made from data, not from opinion, and it happens before anyone designs the new site.
The content audit
Start with a spreadsheet of every URL on the current site, pulled from a crawl and the sitemap so you catch orphan pages nobody remembers. Then add three columns per URL: organic entrances, form submissions or calls attributed to it, and backlinks pointing at it.
Now the sorting is easy. Pages with traffic and conversions are keepers, and their copy carries over largely intact. Pages with traffic but no conversions get kept and improved, usually by adding a clear action. Thin pages covering overlapping topics get merged into one stronger page. Everything else retires.
Resist the instinct to rewrite a page that ranks because the tone feels dated. The words are part of why it ranks. Improve it, do not replace it.
Redirects done properly
Each retired URL gets a 301 to the most relevant surviving page. The closest match, not the homepage. Bulk redirecting to the homepage is the fastest way to lose the value those pages accumulated, and it is a bad experience besides: someone clicked a link about a specific topic and landed on a generic page.
Two redirect chains are one too many. If A redirected to B years ago and now B retires, point both A and B at the final destination.
The blog, PDFs, and old landing pages
Blog posts are often the largest source of organic traffic on a site and the most commonly forgotten in a migration. Move them with their URLs intact if you can. If the URL structure has to change, map every post individually.
PDFs get indexed and linked, so they need redirects too, and anything with pricing or outdated claims should be replaced rather than carried forward. Old landing pages need a check against running campaigns before they are deleted, because an ad pointing at a dead URL will keep spending until someone notices.
Finally, keep the old site's analytics data exported somewhere. You will want the before picture when someone asks whether the redesign worked.
Related: How do you protect lead flow during a redesign?, Can you redesign our existing website?, Is SEO built into your custom web development?, and our blog post on a free broken link checker.