HI, I'm Manoj from freelancer.. We have completed over 150 plus joomla upgrade here in freelancer from joomla 1.5 to 2.5 and 3.0..
We can work on your website upgrade at the sub folder without affecting the live site, we would have 0% downtime.. as a result of the upgrade we will have same website replicated in the latest version of joomla, also maintain same(URL, layout and other SEO)
Please check our review and rating from our client @ https://www.freelancer.com/u/velsoftconsult.html
please let me know..