Google's John Mueller was asked if changing the design of a site but leaving the content and URLs the same would possibly lead to ranking changes. John Mueller said yes, it likely will because he said "if you change design, you generally change the content."
John explained on Twitter "If you change design, you generally change the content: headings, titles, images, internal linking, URL structure, accessibility, speed, etc - they're all critical parts of a site and I'd consider them as content too."
So even if you think you are making zero changes to anything outside of how the page looks to users, there are probably changes that Google will pick up on in terms of your internal linking, accessibility, page speed, content positioning and much more.
Here are those tweets:
Sure -- if you change your site, search will take the changes into account over time. You can't avoid that, and in many case, you explicitly want the changes to be taken into account (for example, if you improve your site).
— 🍌 John 🍌 (@JohnMu) April 18, 2021
If you change design, you generally change the content: headings, titles, images, internal linking, URL structure, accessibility, speed, etc - they're all critical parts of a site and I'd consider them as content too.
— 🍌 John 🍌 (@JohnMu) April 18, 2021
You can still update your site's design and expect minimal ranking changes if you do your best to control everything else in terms of content. So do not be afraid to update the site's design if you have a good SEO helping you along the way.
Forum discussion at Twitter.