The whole bodyshell is new (three door, they only sell 5 door cooking models) with some bits from some modular floorpans (corolla rear end), widened track, etc.
I was gonna say "it's in the article" but it's actually in the 'first drive' linked within the article
It looks like a regular Yaris that has spent several months gulping down protein shakes. It shares a few things in common with the forthcoming new Yaris: it’s built on the same basic platform and has the same wheelbase. But the upper body is all new and the differences are easy to spot: it has three doors whereas the standard Yaris is five-door only, the roof is substantially lower at the rear louvre and the rear features a notably wider track, with big, beefy wheel arches.
Those changes are to aid the design of the next Yaris WRC, due to rules that bodywork must closely follow that of a production car. The rally team gave Saito “many difficult requests”, principally regarding the car’s aerodynamics, weight and strength. Saito then had to battle to gain approval for those changes – and it wasn’t easy, given that Toyota is still essentially a high-volume producer of mass-market cars. “When we asked for three doors, everyone disagreed, because it meant a new body,” he says. “We had to fight for it, but we finally got it.”
it does seem like GR are pushing for it to be a serious weapon - you don't get a whole new bodyshell if you're just sticking some wings on it, a fat engine under the bonnet and calling it 'good enough'.
