The format of the output scope is initially the same as that of the input, but you may set it to a different format, which will convert the node. For more information, see AVAudioIONode.
But if you connect an inputNode to a mixer and override the format to a different sample rate then it will fail. So is this expected behaviour or are the docs not detailed enough? Changing only the number of channels, does not fail for instance. I know you can insert a mixer to convert sample rate.