'lo all:

I have a couple of new IP adresses, and I would like to convert one of my name based sites to an IP based site.

How can I go about this without losing the site data?

Oh, and to complicated matters, my server name is ns.mydomain.com, I want the new IP to be www.mydomain.com.

Thanks!

Just goto site settings as the admin user and edit the IP. The account will then be accesible via that IP instantly. However you may need to wait a few hours for dns issues to update.