How does the signal change cause a current change in this circuit?
In the ideal world, it can't &mdash; but the voltage can still change.

In a common-source circuit, the output voltage is a function of the effective resistance of the current source (in parallel with any load resistance). With an ideal source, that resistance is infinite, resulting is just two output states. But in any real circuit, the effective resistance is very high, but finite. This high load resistance creates a high voltage gain for the amplifier stage.