Sometimes people believe that putting a 20A load on a 30A circuit can cause too much current flow and blow the device. This is not true normally because the 30A circuit breaker is that rating to protect the wires from exceeding that current. The AC unit will take only the current it needs.
That said... As the AC unit gets older, it can draw more and more current. Sometimes "hard start" capacitors are put into the circuit to help the compressor start. So, if you're trying to protect the compressor from exceeding its nameplate current, you could put a 20A breaker in place of the 30A but as "D" says, it may trip more often due to the startup current. In the end, when it's time for your compressor to go it will exceed even the 30A breaker rating and blow it.