They don't control the upgrade: new miners can simply come in to take the free rewards on the new coin if the existing ones refuse to do it. All that matters i

They don't control the upgrade: new miners can simply come in to take the free rewards on the new coin if the existing ones refuse to do it. All that matters is the market's valuation of the currency: it would be irrational for miners to do anything except expand until their costs nearly match the reward.