switching from one network interface to another without reboot

[Michal]

Hello,

How can I switch from one network interface to another without rebooting 
my system (7.2R)?

Problem description: I've got a laptop with two network interfaces 
(wired em0 and wireless ath0). Every now and then I have to set up a DSL 
wireless box which comes with default settings so that I have to start 
with connecting my laptop via em0.
em0 gets IP address from wireless box by DHCP. I log in to web interface 
and set everything up including WLAN and restart wireless box. At this 
point I would like to switch to ath0 and start using internet connection 
via wireless box.
I'm taking em0 interface down with "ifconfig em0 down" and unplug the cable.
I'm changing /etc/rc.conf entries to:
ifconfig_em0="NOAUTO"
ifconfig_ath0="WPA DHCP"
Then I'm doing "/etc/rc.d/netif restart" and ath0 gets IP address via 
DHCP and is connected to wireless box (/etc/wpa_supplicant.conf is set up).

But whenever I try to ping anything (including wireless box) I get 
"interface down" message because my system still tries to use em0.
What else should I do to tell it that I'm connected to network via ath0 now?

Any suggestions are welcome.
Michal
-- 
"The quiter you become, the more you can hear." -Ram Dass