I'm assuming anyone interested probably saw the news already, but just in case, the big N seems to have enabled signups already. This is the account system that supposedly unifies things and finally allows your purchases to be account linked. Sign up located here. FYI, if you sign up with your Nintendo network ID, it adds your purchases from the Wii U era to your history. Looks like you can link a Nintendo network ID later as well, but just to be safe you might want to do it up front.
Thanks for the heads up! I noticed that someone born in 1888 can register, assuming they are still alive.
Yes, the Nintendo Network ID is the same between 3DS and Wii U, so it will work for signups. Not sure if it imports your purchase history, but I don't see why not.