Switching from subdirectory format to subdomain format.

Hi, I currently have a live multisite install using subdirectories (www.mainsite.com/secondarysite) and need to change it to subdomains (secondarysite.mainsite.com).

I'm wondering if there is an easy way to do this, or if I need to start from scratch.

The main reason I'm doing this is that I plan on changing hosts in the near future and the managed ones seem to only support the subdomain version. It appears the subdomain version is supposed to be faster as well, which is part of my reason for changing hosts.

Is there an easier method than starting from scratch? Or do I just need to bite the bullet and get to work?

Thanks for any advice on this.