/etc/network/interfaces not being read?

I'm installing a Proxmox 3.3 host machine which will later go onto another network. So, I need to be able to change its IP address.

I thought I could just put the new network config into /etc/network/interfaces. However, when I reboot, I can't get through to the management interface on the new IP. I have to use the old address.

Is there some other config I need to change as well?
 
It's the default - I simply want to change the IP a a test - it's on the same subnet for the moment:

auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 192.168.1.203​
netmask 255.255.255.0​
gateway 192.168.1.1
bridge_ports eth0
bridge_stp off
bridge_fd 0

Oh, wait, can I do it from the GUI? It looks like I can do it from the vmbr0 entry in the host's network tab.
 
It's the default - I simply want to change the IP a a test - it's on the same subnet for the moment:

auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
address 192.168.1.203​
netmask 255.255.255.0​
gateway 192.168.1.1
bridge_ports eth0
bridge_stp off
bridge_fd 0

Oh, wait, can I do it from the GUI? It looks like I can do it from the vmbr0 entry in the host's network tab.

eth0 is used but missing
yes gui can setup all net config

see http://pve.proxmox.com/wiki/Network_Model

Marco