As new design trends develop, an 'old' website that looks good and suits its purpose can acquire a classic charm. However, there may be compelling business reasons to upgrade.
Firstly, having regular new content is increasingly important – for visibility in Search Engine listings and to maintain interest amongst your customers. A modern user-friendly Content Management System will allow you to keep your presentation up-to-date easefully.
Secondly, techniques and expectations have developed. What was standard practice a few years ago may not be so efficient today – or even display properly on modern systems.
In particular, clear and informative signposting and navigation is crucial. As is human-friendly and 'scannable' content.
Thirdly, your own mission has developed and should reflect your present quality, visually and functionally: i.e. a more integrated design, with more interaction. And it's a good opportunity to review content thoroughly.