Aside from the lower resolution, the game is mostly the same. Outside the main game, there is one major addition. The game has a music player and model viewer. The player gathers them as he plays but he can get them faster if he streetpasses with another player or uses the Shulkamiibo. The music player also works when the 3DS is closed like in Super Smash Bros. for Nintendo 3DS so the player can hear the soundtrack on the go.
The game was revealed in Japan alongside the New 3DS on August 29, 2014. They stated that the game was complex to run on the old 3DS. It wasn't until a Nintendo Direct in January where Nintendo revealed it for the West alongside the US reveal of the New Nintendo 3DS XL.
They also announced Monster Games was the developer. After a Japanese-only stream of Xenoblade Chronicles X, Nintendo opened the official site where they revealed the Streetpass and amiibo features and this same info was revealed for the West later that morning with a release date for North America.