replace 20A switch with 16A switch
Replacing a 20 A switch with a 16 A switch without knowing anything else is a REALLY BAD IDEA.

Chances are you won't burn your house down, but how lucky do you feel?  Fortunately, 25% overcurrent thru a switch is unlikely to do more than burn out the contacts such that the switch either won't close properly anymore or weld the contacts shut so that it is permanently on.  If the switch is in a metal box so that nothing too bad would happen if it reached the melting point of plastic, you probably won't burn your house down.

Even if the space heater doesn't draw more than 16 A, you still have a problem.  Suppose stuff happens (which it regularly does to space heaters, they are one of the leading causes of house fires and electrocutions) so that the power wires get shorted.  You want the breaker to trip to shut off the circuit.  This might not happen if the switch is a weaker link than the breaker.  Instead of the breaker popping, the switch might catch fire.  In general, it's a good idea to use a switch rated for at least the trip current of whatever power segment it is on.

To paraphrase Dirty Harry: Do you feel lucky?  Well do you, punk?  Go ahead, use a 16 A switch.  Make my day.