Hey there Andrea,
Search engines focus more on your content and site structure than design, so you want to focus on consistency in this area.
Keep your 1,700 articles intact and at the same URLs.
Make sure none of your current pages (pre-redesign) show “not found” errors after the redesign.
If you do end up changing any URLs, make sure the old URLs perform a redirect (HTTP 301) to their new counterparts. Note that your pages will still lose some influence due to the redirection, but it won’t be anywhere near as severe as if you failed to redirect the page.
Avoid changing your site’s navigation elements (links that appear on most if not all pages) if you can help it. These assist search engines in determining the priorities of your pages, after all.
Sign up with the Google Webmasters Search Console before the redesign. They’ll regularly monitor your site and will let you know if any issues arise after the change. They’ll even give you advice on how to fix them in many cases.
A good way to handle this is to imagine that your visitors have bookmarked every page on your website in their browser (these bookmarks go to specific URLs). You want to make sure that if they click those bookmarks in the future, the content they expect to see will still be there, even if it requires you to redirect them.
Changing your theme won’t automatically change your site’s URLs (permalinks) but make sure any theme-specific shortcodes and such that you’ve been relying on are updated throughout the site.