Yes, you can use the same friend code as long as you transfer the wifi settings BEFORE you go on WFC. To transfer the wifi settings, just go in the wfc settings in any wfc enabled game (on the lite), go to options and transfer nintendo wfc configuration. Confirm that you want to transfer the settings by pressing A, then go on the DSi, press download play and download the settings. Note that the wfc settings get deleted from the old DS after that, so you can't use the same friend code on the old DS anymore.