"what happens if you inflate nitrogen-filled tire with regular air"
I'm assuming they actually drain the regular air out and re-fill with nitrogen. However, I'm guessng they do just that, "topping up" with nitrogen.
Never used nitrogen in my tires. I heard there is no real difference. My mechanic told me to save my money and use regular air and check the pressure every 1-2 weeks or before a long road trip (eg. driving to USA).