He's not just changing for the sake of change; he's trying to diversify the farm's output, rather than relying on a single source of income. The cows are going to still be around and the wheat is still there every year.
If you took nothing else away from the show, you should have taken away this: Farming is not profitable. At the end of his first year, his profit was just barely a few hundred quid. Sticking to just the grain might have netted him a few hundred more, but the risk of that is that one lean year can leave you with insurmountable debt. It's the reason so many smaller farms have gone under in the last 20 years. It's the reason he's become such an advocate for farmers and farming and why he made the show to begin with.