A lot of websites have done a redesign lately (mostly moved to WordPress), even some websites that I’m doing SEO for.
What I can say is that 90% of the redesigned websites have suffered a big ranking loss in the following 6 months after the complete redesign. This happens because of the following reasons:
Indexed pages urls are renamed without being 301 redirected to their new version.
Basically Google expects to see the same url name for the new page, same meta titles and meta descriptions, same text content, the same images and same alt tags of images.
Some indexed pages(or a high percent of them) are completely deleted.
Conclusion: in order to prevent most of the ranking loss the new version of the website will be almost undistinguished from the old version in the Google robots eyes, or as it can be seen as a html published page source. Especially when it comes to text content and images, alt of the images, page urls, title tags, headings H1, H2, H3 and H4, meta description tags.
Google doesn’t care if the new version has different or removed scripts, different internal coding, divs or classes for the same page as long as the title, the meta tags, the headings, the content and the images are the same.