Most people I know who still use CyanChat don't use it directly; they usually (me included) chat from the IRC relay or using an external program. This has the advantage of letting us be always online. And so far, it still works great for something which was written as a test case.
Still, since the Cyan website has now been updated to look more modern, it might also be a good idea (if you think it might attract new people) to update at least the
website, which really looks dated. And if you update the website, it might be a good idea to update the Java client too.
If you do choose to update CyanChat, maybe it would be simplest to switch over to IRC (which a lot of people already use) and create a special client in Javascript with AJAX? I'm hardly an expert on the matter, but my favourite chat programs online so far have been Javascript-based, since they're lightweight and don't require any external plugins.