Aaron Parecki

No, that's not what I meant.

Only the order of the channel IDs specified will be changed

In your example 1, c has moved as well, even though only d and b were given.

To move a channel up or down...

This is equivalent to swapping two adjacent items.

I didn't give an example of setting the order of three items because I couldn't think of a UI where it would make sense, but it is still possible.

The nice thing about this approach is that the same update logic works for all the use cases, and doesn't matter how many items are in the list, and is atomic.