Magento 1.5.0.1 Upgrade to 1.6.1.0 Community Edition

I need my website upgrading from 1.5.0.1 to 1.6.1.0 - It's using a custom theme (I have the theme updated for 1.6.1.0) and I need the styling to be the same (I only changed CSS files to re-skin the theme). I need this turning around in 24 Hours with a 503 page telling potential customers to come back later and it can be done on a live server (I have everything backed up). The site needs to look and function exactly the same as before the upgrade with the addition of the new theme functions. Acumen is the theme ([login to view URL]) I will give bidders the URL when requested.