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There are a couple options that I see here: Go through the list of pages, find the most relevant equivalents on your current site, and 301 redirect them. Yes, 20,000 pages is a lot but you'll quickly fall into a groove where you find whole sections of pages that can be redirected to the same place. I did it with 65,000 or so URLs when SEOmoz moved to Moz, and it took about a month to plan out alongside my other tasks. It's tedious, but doable. Figure out which pages have the high-value links you want to preserve, 301 redirect those to their most-relevant equivalents, and redirect the rest to the home page. This won't work quite as well to preserve link equity or any traffic/rankings they might be getting, but it will be faster. Either way, once you've got those redirects in place I strongly recommend taking a look at the sites that link to the old pages, and seeing which ones you might be able to reach out to to get the links updated. That way, you won't be losing link value through the 301 - plus, you might strengthen or re-establish some relationships that could result in future links. Good luck!