I think the NX and future unified platform will use the same online infrastructure, only building upon it. So effectively never. Or at least, not for a very, very long time.
That's an interesting point. Though I expect NX to have a new online infrastructure even if it runs WiiU games. It's not that I think Nintendo will start charging money for Online play on the NX like Sony and MS are doing with their new versions of PSN and XBL, but there's always room for imrovement regarding the online network. And sticking with the last gen one seems like a possible long term mistake in that regard.