What can /chotbar copy do that /chotbar change cant? With copy, you're using an extra bar to mediate between 2 or more bars. With change you just jump straight to the bar?

Like the black mage example, for example. You use a 3rd bar and then directly modify that 3rd bar by taking copies of set 1 (fire) and set 2 (ice). With change, you would only need the transpose macro on set 1 (fire bar) to tell the bar to change to set 2 (ice bar) and then do the reverse on bar 2 (ice bar) by telling the transpose to change to set 1 (a second macro, still the same with your change setup). Then you'd only switch around between set 1 and 2.