With the recent release of the DS-Lite in Europe, many people will be upgrading to the newer, sleeker model. One problem that can arise is setting up a new WiFi account on your new console then having to let all your friends know your new friend code(s).
Well there is infact a function built into WiFi game that will allow you to transfer your WiFi settings to your new console:
On your old DS with any Wi-Fi game inserted, turn it on and go to the Wi-Fi settings (where you go to search for access point)
Go to OPTIONS, you'll see three options here, the last being "Transfer Nintendo WFC Configuration" which you want to select.
Turn on the DS Lite and use the "download play" option and it should transfer the "WFC ID Number" from the old DS to the Lite (The ID will be erased from the old DS).
Remember that if you're using a home router to connect and it is set to only allow access to your old DS, you'll need to update it with the new address of your Lite.