They will not fit you as well as your natural size. The angle of the cup will be different, and the situation of the cup on the band will be slightly different.

For many women, however, the "sister size" (up a band size, down a cup size) will work well in a pinch.

But if you can't get the bra hooked because the band is too small, the cup size is somewhat inconsequential.

Of course. However, if you were the 36C in the previous example, a 38B will work.

Again, this won't work for everyone. But it does work for a *lot* of people.