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I work for a company that closely mirrors the dropdowns employed by a very large ecommerce site - http://www.surlatable.com/. Upon analysis of this URL, there are well over 100 links on that page because of the way their dropdowns are designed (over 400 in fact). However, upon close analysis, only 52 of these internal links are followed, and 370 internal links are not followed. Based on this, I would recommend if you have well in excess of 150 or so links, use nofollow for those less important sub categories like this site has. (I'll have to change the structure of our site now...)

Well, I think in many cases change is good. I understand your concern though. I think redirecting the current url will be fine and you won't lose traffic. They are still clicking on the old url, and landing on the new. I don't think that will be too off putting for anyone based on experience.  We do 301's from totally different sites and still have the traffic when they land on a page. As well as that, you won't lose any link juice to speak of as the 301 will transfer 90% plus. I would use the OurDomain.com/keyword or keyword-phrase as opposed to having the .html extension after it. And, dashes as opposed to underscores. Best Edit: Forgot your last paragraph. Be careful with adding pages of similar keywords like: Purple Monkeys to Big Purple Monkeys, etc. You don't want to be so close that you impact one keyword while thinking you will move the other up. Remember that not every search is exact match.  Look at where you are failing in conversions: Is it the ranking not bringing you traffic or is it you are getting impressions without click through (meta description maybe?) or is your traffic bouncing because your landing page sucks or is not answering their query. Best