If your company site is based on a complex, open-source application, it’s live, and you don’t want to put a copy on a development server - create an alternate view or access path.
Any company that does or allows development live on their own site (easy to see with errors, browser incompatibility issues, layout problems, IE conditional tags which are displayed) risks their credibility, particulary if they are in the web business.
Create an alternate access path into the application and update the skins and modules as separate components accessible only through a development path until new code has been carefully reviewed for quality. Most powerful content management systems have some method to identify a site visitor entering through a different path. You can use a second domain name, a subdomain, a different URL or port. You may also need to modify the application very slightly.
The technical team (designers, developers and engineers/backend people) should build their code off-line and outside of the application, then integrate it. Complex applications have longer load times than simple pages, and it is very easy to make a mistake that ripples through the entire page.
It may be better to take the site down for major upgrades. Doing this after hours should avoid noticeable service outages.