If you want to continue playing Pokémon Black Version, Pokémon White Version, Pokémon Black Version 2, or Pokémon White Version 2 on a system from the Nintendo DS family (and not on your system in the Nintendo 3DS family), you need not worry—there is no need to transfer your Nintendo Wi-Fi Connection ID.
On the other hand, if you expect that you will continue playing Pokémon Black Version, Pokémon White Version, Pokémon Black Version 2, or Pokémon White Version 2 on your Nintendo 3DS or Nintendo 3DS XL, then you will need to transfer your Nintendo Wi-Fi Connection ID.
Please note: Nintendo Wi-Fi Connection service for Nintendo DS ended in 2014. The discontinued services include online play, matchmaking, and leaderboards for many Nintendo DS and Nintendo DSi games. Users can still play the games in offline mode. Online play for Nintendo 3DS games is unaffected by the change (aside from Nintendo DS games on the Nintendo 3DS family of systems).
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