how to upgrade from NetBSD 3.1 to 4 ?

I read the documentation on the netbsd.org site and only suggests one upgrade method, that is to
boot from a NetBSD 4 CD and upgrade that way.
The problem is that my broadband firewall / web server / mail server is running NetBSD 3.1 and has
no keyboard, no monitor, and no CDROM drive. It lives in the cupboard under my stairs. I only access
it using ssh.
Is there no other way? like put a netbsd 4 kernel on the drive and reboot it or something
Booting from CD is going to cause quite a lot of hassle.

thanks for any help, Philip