Mitsubishi made a joint company together with GS Yuasa, so they are more connected than just the usual buyer/seller relationship. I suspect they will stick with the existing batteries.
That being said, Mitsubishi does use Toshiba batteries in some markets for their MiEV (and the GS Yuasa in others), so it is not impossible that they could switch or even have different batteries in different markets.
I do hope they use the Toshiba batteries in my market. Those batteries charge about twice as fast and they have much longer life (at least 2.5x as many charge/discharge cycles).