It seems that without an official announcement, Nintendo is in the process of discontinuing the original 3DS model.
As Destructoid notes, the handheld is no longer sold directly through Nintendo's website, while retailers in both the UK and the US are reporting shortages. One peripheral manufacturer has categorically stated that the 3DS is no longer in production.
Both the 3DS and 3DS XL have already been discountinued in Japan, where the New 3DS is already available, and it now seems likely that the models are being removed from production in the West as well. That said, this is all speculation at this point and the New 3DS is not on sale in Europe or North America – Nintendo has yet to reveal its planned launch date for the system in those regions.
If the original 3DS and 3DS XL are being phased out, it's likely those who own the systems will need to upgrade if they want to play new games, as the New 3DS is a more powerful system and its titles are not playable on the older handhelds. The 3DS XL is not currently experiencing stock shortages, and the 2DS remains available.
Nintendo has yet to officially comment on the reports.