A good rule of thumb in Linux is "don't fix it if it isn't broken"
In other words, if your KDE version is running fine, and you have no problems, then "upgrading" isn't necessarily a good idea. Is there a specific feature or need in KDE 3.5.9 that you are currently missing? Newer isn't always "better".
Remember Linus Torvald's immortal words: "How the hell should *I* know if it works? I just coded it. That's what we have users for."