Nintendo has done something almost never done in the game industry (except perhaps by Nintendo themselves): they've updated a console mid-cycle.
Most consoles will go through re-vamps over the course of their lifetime -- new colors, smaller bodies, etc. But those new consoles seldom have functionality that breaks compatibility with the older systems. For example, the XBOX 360 S and the XBOX 360 E have different form factors and built-in Wi-Fi networking, but they still play the same games as the original XBOX 360.
In the case of the new 3DS XL, Nintendo has upgraded the processor in the system. This means better performance, but it also means that games written to take advantage of this faster performance won't be compatible with older systems. Right now the only game made exclusively for the new 3DS is Xenoblade Chronicles X, which won't work on the older 3DS systems, but there may be more such games in the future.
It's hard to imagine that most game makers will ignore the tremendous installed base of 3DS systems (currently about 44 million) by making games that only work on the 3DS, but Nintendo will probably want to showcase the system and give gamers a clear reason to upgrade. Third-party developers may decide to create games that work on both systems but give some extra oomph when running on the faster devices. This would be similar to cross-platform games that work on the XBOX 360 / PS3 and the XBOX ONE / PS4.
Unfortunately, North American gamers will only be able to buy the XL version of the handheld initially; the normal version won't arrive until an unspecified date in the future. It probably won't be necessary to upgrade immediately, but if you like 3DS gaming, there may eventually be a title that compels you to upgrade at some point in the future.