Do this one piecemeal.

Start with specialized functions (eg move the fileserver over) and applications (eg let someone give openoffice a trial, swap out IE for mozilla, etc).

Do not, and I repeat, do not try to migrate desktops until you have as much else cut over as possible. When you try to switch users, you want to be familiar with the OS (from setting up servers) and you want users to be comfortable with the applications (from using them on Windows).

Good luck,
Ben