Adding new IP range - redirect traffic out through other interface as default route

hey Guys,

i am getting a new IP range for our office.

i already have the "old" range on eth0, i want to configure the new range on "eth3".

Once the new range is configured, how can i route all network traffic out over eth3 by default instead of eth0 ?

i basically want to do this switch with 0 downtime.