Websites, like most things, can get stagnant overtime if there are little changes made to it and there often comes a time when you need to have a complete overhaul of your site. Some companies chose to do this in stages and improve sections of the site bit at a time, but when doing this you must take extra care to ensure that the site doesn’t look disjointed and that all the elements still work correctly with each other as they should do in the mean time. If you have the funds and the man power it is often better to put up a basic site for a few days and get the changes done to the site in one go. Ideally your programmer should take a copy of the site and work on it offline so it doesn’t disrupt the new site.
Conversion rate is often a reason why many businesses decide to update their website. It may be that the search engine optimisation element is working as it should and you are getting a lot of traffic to your site, but that traffic is not converting. If this is the case you need to have a look at things such as the bounce rate for each page and how long people are staying on that page. This will give you an indication as to where things may be going wrong.