This is by no means an exhaustive list of reasons why you are playing russian roulette when you redesign your website. By redesign I mean changing your theme, changing your URL structure, or even what would seem minor changes like inserting tags into your template, swapping your navigation menu from left to right, or adding a little boiler-plate wording to every page. Now everything I'm about to say is theory and my own personal experience with changing templates. I've lost a site that made over $100 a day which never regained its rankings after a year. I also have a good friend who lost so much money over a site redesign that he went back to being employed by someone else (a leader in his niche, physically stocked and shipped over 5000 products). #1) Different URL Structure Right now I'm talking about 301 redirects which alter your URL structure. 301's do not pass full pagerank or link value. When you get backlinks, Google assumes someone linked to you based on the merit of your content. If your URL changes, and the content changes (via a different template), perhaps Google may think that person wouldn't otherwise have linked to you. Although they admit that PR isn't being passed, its fully possible that other things aren't being passed, such as the authority or the full benefit from the anchor text of the link. It can take Google months to show your proper internal linking structure in their webmaster tools. Even though they may spider your site in a week or two, it takes them months to analyze your content, your internal linking structure and to post that to your webmaster tools. When all your URLs change, your internal link portfolio is dropped. It could take many months before Google knows which URLs internally point to which URLs again. Many websites rank based off nothing but their internal URL structure. To totally have your internal link portfolio disregarded by Google for a time will drop you in the SERPs for certain -- some people would call that a Google dance, but in reality its because you lost all your internal link credit. 301 redirects are abused by webmasters. When Google sees a 301 redirect, it doesn't automatically assume that the new URL is the final resting location of the old content. Most websites try to preserve their PR and "link juice" by 301 everything to their homepage that is 404. Use Google FTW. help rank dropped after changing theme. I mean COME ON -- even if you read SEO forums/blogs for a minimum of 5 minutes a week you have to see everyone complaining about changing their theme has caused a drop in rankings. I think everyone would agree that as a web page ages, its rank in the SERPs increases. Does age get reset when its changed to a new URL? Theorize whatever you want to, my theory is YES the age gets reset and now you're stuck having to re-age the content of that particular page all over again. No one can prove me wrong (or right), but it is very plausible that it could happen this way. #2) Different Keyword Density Ratios Finding the "sweet spot" of the proper keyword density is a waste of time. The most optimal keyword density is not going to give your page a bonus -- you either have the keywords or you don't. The difference between a 1.5% keyword density and a 4% keyword density may have helped in the past, but not any longer. However -- when it comes to keyword density, you absolutely have to be concerned when it comes to stuffing. Penalties for keyword stuffing (otherwise known as overoptimization) are very real and depending on the overall trust/age of your website could be enough to give your page a drop in the SERPs, have your individual pages deindexed or get your site sandboxed. There are many sites that may be on the boarderline of overoptimization, but are fine as-is. By overoptimization I mean they have spammy backlinks all using the same anchor text, keyword stuffing in the pages, too many keywords in h-tags, etc. So then you change your template by adding in a "tags" widget. This proceeds to inject say 40 hyperlinked keywords on every page of your site. If your website was boarderline being penalized for being overoptimized (but still ranking in Google) -- then you add in this kind of tags widget that pushes your keyword density over the limit, half of your website could take a nose-dive in the SERPs or get deindexed from Google. #3) Your Site is Completely Different = Site "Start Over" in Google The assumption that people make when completely redoing their old URL structure and morph it into a new URL structure, they think Google will consider the new to have the exact same PR, same authority, same anchor text weights, same age benefits, and consider your new URL structure to be a newer, better version of your old URL structure. What I think happens is you will get a tiny bonus on whatever you were trying to accomplish (changing from dynamic variables in URL to fixed URL structure that has keywords in URL will give you a slight bonus), but at the expense of taking a partial loss in PR, partial loss in authority, partial loss in anchor text weights, and a full loss in age benefits for that particular page. #4) Other Mix-Ups Some people think that the first link on the page is the only link that counts, at least according to some SEO professionals. Other people think that in-content links are more valuable than navigational menu links. Well, what if your navigation menu was in your right column, and you switch it to the left column? All of your in-content links linking throughout your site used to be seen first, because your navigation (being in the right column) was listed after the links found in content. Then you switch your navigation to the left column. Suddenly, your less-valued navigational links come before your in-content internal links. As a result, your entire internal navigational scheme has been completely devalued (in theory), because you no longer have in-content internal links that are recognized by Google, only navigational links. When should you change your template? You can change your template if ... You are not changing your URL structure. Your changes are designed to reduce keyword stuffing issues. Your changes are designed to enhance your internal linking between related pages (such as with the YARRP plugin). Your changes are not significantly adding to the content of your pages, necessitating a Google reevaluation of all your content. A website will have more ability to make larger changes if it has higher trust in Google, greater age and better quality backlinks. I might be wrong with a significant portion of what I posted above, because (as with everything SEO related) all you have to work with is theory and assumptions made by past experiences. But I fully believe that screwing around with your site's template can seriously mess up your earnings. I never change templates anymore except under the conditions I listed above. Bottom line is this. If it isn't broke, don't fix it.