By the way, the account ID is internal to the system. So, while your email address that you register to the account (which you log in to the site with) must be globally-unique, you can change it to something else if you no longer have access to that.
For example, I created an account so that I could link my Japanese NNID to it. However, it was my main email account. So, when Nintendo rolled out the account system to NA, I registered my secondary email account to the existing Nintendo account, and created the new Nintendo account with the email I released so that I could create one for my NA NNID.
I am curious then if they will eventually change the way usernames are handled if they decide to deprecate NNIDs or merge it into this.