Many sites go through redesigns and in the process URLs change. In particular, most sites we redesign we attempt to use pretty urls that contain keywords for the page. The keywords end up bold in search results if they are in the search terms. The thinking is that the bolding of the keywords increases your CTR on search results. The problem comes when you try to figure out what to do with a site that is already ranked well and you are switching over to pretty urls. The question is whether you should let the old urls just 404 or should you go through and make 301 redirects to the new pretty urls. Here are my thoughts on it:
The search engines will see the 301 redirects and start to modify their index with the new urls. The entire site won’t be reindexed at once so for a period of time it will still think that there are current pages on the site linking to urls that then do a 301. This will negatively impact rankings. How much impact is difficult to say. As the entire site is eventually reindexed the search engines will see that the site no longer links to the old urls and the site won’t be negatively impacted. I’m guessing it’ll take 3 months to remove any negative impact. Its just an educated guess though.
The 301 redirects will also have a positive impact on rankings in that the urls will contain the relevant keywords. As the site is gradually reindexed, the search engines will start to see that there is a navigational importance to the keywords used in the links on the site. It will boost the effect of those keywords and improve how the search engines interpret the site’s internal linking structure. As a result, it will have a positive impact.
Once the site is completely reindexed and the negative impact from the 301 redirects is negligible the site will ultimate have better ranking potential than before the change since the internal linking structure has been given more weight to keywords in navigational structure.
This is all known and predictable. What we can’t be sure of is if the weight of the positive impact will balance the weight of the negative impact in the short term while the site is being reindexed. From my experience, I expect rankings to slide initially and over a three month period regain and surpass the original rankings.
If you simply ignore the current site urls and just do 404s, the search engines will start to reindex your site but will also register a bunch of 404s. This is probably going to give you a much more negative impact than you’ll get from the small decline you’ll get from doing 301s. I strongly advise doing 301 redirects and to remember that your rankings will likely drop slightly but will ultimately recover and exceed where they were before (that’s assuming there wasn’t any changes in the redesign itself that negatively effects your rankings).