Well they can start planting now. I think the main possible issue with switching is if the current producers are producing enough for their current market. Like, the Americas produce corn for their consumption but we don't know if they'll have enough excess to sell if the people feeding on wheat switches to corn suddenly.
Wheat is planted in winter and it gets ready in summer. Wheat is not a winter crop.
How ever I dont think this would happen. There will be shortage - this is just a panic created. I am sure that this food crisis will not happen. Ameen