We were working on an established site the other day.   It was an old client who had left us, but came back.  They redesigned their website and all of their rankings had disappeared.  Luckily for them, they are quite established (their site has been on the web for 10 years or more) and the specific keywords were not outrageously competitive - and so our changes were implemented and made live and they began receiving top 3 rankings in Google for most of the core keywords within 48 hours.

This is a great example of how "fresh" the Google index is and also how quickly things can change.  Just as quickly as we were able to get our client re-established, some other site dropped as we took their place. 

The SERPS are constantly changing - especially for competitive keywords.  Google and others want their results to be fresh and reflect content that is not only relevant, but current.  This is why it is important to ensure that your website is also fresh. 

Once upon a time, you could create a web site and leave it untouched for years - today, this would be perilous if you expect any sort of traffic from the search engines.  And as you have seen above, don't forget about SEO when you redesign your web site - why spend several thousand pounds on a new design and then even more on implementing it... only to ignore your SEO company and disappear from search.