No, re-designing it would actually help it because it's showing Google that you're updating your content and prividing a better experience for the user - assuming that your re-design is actually a great design. I would recommend using WordPress. It's similar to Joomla, but much better and you'll find more design options. WordPress has a lot of great themes (templates). As long as you're not deleting content and pages - then you're rankings will be fine. Just make sure you do it within a few days time. You don't want a website in progress for a month. But the link juice flows to the domain, so you're fine.