The Internet is constantly evolving in terms of both the technology that powers it and the design trends of websites.
Whilst at the time of launch a website may use the latest technology and cutting edge design trends it will not remain so indefinitely. Over time design trends change and what is today a cutting edge website will reach a point where it looks dated and stale and in need of a refurbishment to keep up to date with evolving user expectations of what constitutes a contemporary up to date website design.
This is part of life and similar design trends govern the look and feel of everything we use, wear, drive and even eat. Over time the the individual pages of a website become an entrenched part of the internet as a whole and the longer a page is online the more weight it is given by some of the algorithms used by search engines. For this reason it is important that these established page entrenchments are considered when it comes to creating a new website to replace an established one.
An ignorant website developer when asked to redesign a website will simply discard the old site and start building a totally new site to replace the old one. The result of this is that in many cases pages that have spent years entrenching themselves into the fabric of the internet are suddenly no longer there and all the search engine visibility that has been built up is lost. Essentially the website is starting all over from scratch, not a very sensible idea.
The sensible website designer will if possible try to retain the actual page URL's of the pages of the existing site and if not possible will setup server side redirects that inform the search engines and pages linking to the page where to find the new page.
The experienced website designer will develop a website using a content management system that separates the actual content of the site is stored separate from the actual display elements of the website so that when a site is due for refurbishment all the existing content and functionality of the site is retained and only the look and feel of the site design needs to be changed to freshen the look of the site and keep it in line with the current design trends. This is the sensible and cost effective way of developing a website to make it durable and provide long term longevity for the site.