Converting a fixed-layout website to a responsive design website can ruin search engine ranking unless the responsive website redesign is done right. On the other hand, a conversion done the right way will usually raise search engine ranking, simply because most existing websites are not particularly search engine friendly. All the responsive websites I make are search engine friendly.
“Search engine friendly” simply means that the site structure allows the search engine bots to easily access every page and post, menus are such that the search engines can figure out where the really important pages are, and the text on those pages is organized so the search engines can figure out which words and phrases are most relevant to the central theme of the website.
The main reason redesigns ruin ranking is because many designers don’t take care to preserve all of the URLs – not just URLs for pages, but for posts, and media too. The first step in doing this right is to make a list of all the obvious URLs such as pages. Next use a tool such as Open Site Explorer to discover all of the inbound links. Open Site Explorer will also list all of the anchor text from back links on referring pages. If that text does not include relevant keywords it’s sometimes possible to remedy that with the cooperation of the referring site. That alone will boost SEO ranking – in some cases a lot.
This list of obvious URLs and inbound links becomes the designer’s reference for naming pages, posts, and any element that has a link to it. In the re-designed site, generally, all those elements must have the same URLs as the old site. When designing a site from scratch it’s important to give (for example) images names with relevant keywords in them. But in a redesign or a conversation to a responsive design it’s generally more important to preserve the file names (and extensions) that were used in the old site.
Following these steps ensures that the search engines will find things where they expect to rather than finding broken links, all-important Google PageRank based on incoming links will be preserved, and humans following inbound links will find what they are looking for, rather than being frustrated, leaving, and never coming back.