We have 246-219=27.
To see why this can't be smaller, notice to get something smaller the hundreds digits should be the same or differ by 1.
If they are the same, they must both be 2. so it just becomes the positive difference between a 4, 6 and a 9, 1. It is easy to see that that difference is 46-19=27.
Otherwise we need 2 _ _ (4,6) - 1 _ _ (2,9). The minimum difference is 246-191=55.
Hence the answer is 27.