You cannot directly "trade in a DS for a DSi" as a 1:1 swap at retail. They are different hardware generations. Here's the realistic process:
| Feature | DS / DS Lite | DSi |
|---|---|---|
| Cameras | None | 2 (0.3 MP) |
| SD Card Slot | No | Yes (photos, music, DSiWare) |
| DSiWare Shop | No | Yes (shut down 2017, but installed apps remain) |
| GBA Slot | Yes | No |
| Screens | 256×192 | 256×192 (XL: 4.2") |
| Browser | No (needs cartridge) | Built-in Opera |
Decide if DSi features (camera, DSiWare, no GBA slot) are worth the upgrade cost over your current DS.