Upgrade directly from 1.5 to 2.5

Hi,

I have the old joomla 1.5 and virtuemart 1.8 versions installed, and i now plan to update to J2.5 and VM2.0.6. Do Gavick have a own way to upgrade the site and system, or do i have to install a fresh 2.5 version with a fresh virtuemart and migrate the users, articles and virtemart products manually? Whats the easist way to do this?

note: i tried to just overwrite the exisiting files (on a mamp localhost), but the site went from working (like the online version), to just white screen. After i overwrote the files i got directly to the joomla 2.5 installation, direcly to the "last step", which stated; delete you installation folder before accessing your site. Nothing works from here on.