Can You Trade Pokémon Directly Between DS and DSi?

No direct trade. The original DS/DS Lite and DSi/DSi XL use different wireless protocols (DS: 802.11b proprietary; DSi: enhanced 802.11b/g with WPA2). They cannot communicate for Union Room or Pal Park transfers.

Official Transfer Path (Gen IV → Gen V)

1. Own a Nintendo 3DS/2DS (any model).
2. Download Pokémon Bank (paid subscription, now free since March 2023) and Poké Transporter from eShop.
3. Insert DS/DSi cartridge (Diamond, Pearl, Platinum, HeartGold, SoulSilver, Black, White, Black 2, White 2).
4. Use Poké Transporter to move Pokémon from Box 1 of the DS game → Pokémon Bank.
5. On 3DS, open Bank, move Pokémon to desired Gen V/VI/VII game.

Notes

• Requires 3DS family hardware—no PC/mobile alternative.
• Items (held items) do not transfer; they go to Bag.
• One-way transfer; cannot send back to DS games.