There are only 10 possible first digits of the new number, so you can't choose a number that will differ from all other numbers in the first digit. If you do it

There are only 10 possible first digits of the new number, so you can't choose a number that will differ from all other numbers in the first digit. If you do it diagonally, you'll always have 9 other options to choose from, since you just have to make it different from that one number!