My oldest site, (circa 2000), needs to undergo a complete change. Everything, including the theme, platform, content, hosting, and design needs to be completely redone. This is a task I'm dreading, not only because it's going to be a fair amount of work, but because of the SEO value that may be lost. The question to be answered is how much SEO value will be lost...if any?
One page of the site will basically look the same, but will direct users (via a manual link) to another brand new site. Products that have been paid products for all these years will now be free for download on the new site. Anyone getting to the products page from a search will be told to download them elsewhere.
The rest of the site will be completely different. I will keep track of any SERPs changes that occur due to the changes and let you know what happens in future posts. Wish me luck!