Its a fact, you will gain more business by keeping your website up to date, that includes the content, products, pricing and the general look and feel of your website. Yes websites too have a shelf life. If your not sure about how current your website is, then just have a look at the competitors in your market, wether a builder or a dentist, an estate agent or a computer repair company, you will see how your website will be compared to trends.
Firstly if your site was done three years ago or more, the site was probably done in straight html, no bells and no whistles or it could have a whole load of 'Flash' embedded graphics. Well good news, thats 'Old school'. More and more web designers are using CMS (content management systems), XHTML, different frameworks for placing pretty much any data anywhere on a page i.e. 'Gantry' From rocket theme (leemaggs.com uses Gantry) or 'T3' framework from Joomlart. All of these make your site so much more socially friendly, with customers being able to post comments on your site, building a social aspect. being able to configure your website to the users taste and stye.
The best part about a modern website is that you can now add your own data or material to a website with little knowledge of web design, its a case of logging in and using a similar interface as a word processor.

You may well impact your position in the search engines short term, your previous customers may well not like the 'new look' or 'feel' of the new website but people dont like change, especially if the are use to your website.
Most modern websites are built much more seo friendly (search engine optimisation) so there may well be a little lag, whilst links from your old site get updated, google and others will index your new site and most probably will be able to do this much more easily.
If your users dont like the look and feel of the website, it's quite straight forward now to add a comments section, or ask using a facebook page about your business. its also quite pleasing to be able to change the sites look and feel using templates and or CSS. (cascading style sheets)
leemaggs.com has upgraded many websites and has encountered a good majority of the problems faced today. Come and speak to us and we can help you transform Old to New.