It's not confirmed when the switch will happen, but with the Renault e.dams Z.E. 17 (seen in testing above) already having been unveiled for next season, it's likely the switch will come in for the 2018/19 season when the series moves away from the practice of swapping cars mid-race.
Renault has been a successful part of Formula E from the very beginning, providing (with partner Spark) the cars each team used in the first season, and scoring the series' first Team Championship in 2015, and then again in 2016 and 2017. Nissan, though, has the globally popular Leaf EV, and has been eyeing an entry into Formula E for some time now. Joining the series will not only boost the visibility of Nissan's electrification strategy, it will do the same for its Nismo arm as well (and, as we've already learned, there could be a Leaf Nismo offered in the future). The change would also free up resources for Renault to focus on its F1 efforts.
As Motorsport also reports, Formula E CEO Alejandro Agag has said of the Japanese brands Nissan, Honda and Toyota, that "one of those three, maybe two, will end up" in the electric racing series, without elaborating further. Nissan isn't expected to confirm its Formula E entry soon, but it could happen at the Geneva Motor Show next year.