Hi there! If I book an Upgrade Award, is my seat in C guaranteed, or is there a chance I could get downgraded to the Economy seat I paid for?

I'm considering booking flights that I only want if I can go business class. I don't have the miles for outright award tickets, but could pay for the economy ticket and upgrade with miles. I'm a little apprehensive that I could get bumped back down to my paid ticket and wind up on a trans-Pacific flight in cattle class, which I really don't want.

Also, will I still earn miles (at Economy level) for the flight?

Thank you! I'm still learning my way around United's program.