"After this process, only New 3DSs which have never accessed the eShop before and all Old 3DSs will be able to access the eShop after creating a new NNID on their new region. Region changed New 3DSs that have already accessed the eShop on their original region cannot create a new NNID and access the eShop on their new region!
Note that occasionally the eShop will inexplicably still not work for devices that it should work for. This is entirely to do with Nintendo’s servers, and I really do not have any control over it. Additionally, note that occasionally the eShop will inexplicably work for devices that it should not (such as some New 3DSs). The cause for this is currently unknown."