Electrical – Is it OK to replace a 15A outlet with 20A outlet as long as it’s already on a 20A breaker

You are correct on both counts:

• There is no problem with using a 20A switch on a 15A circuit, as the breaker will trip and protect the switch from over-current.
  • Using a 15A switch on a 20A circuit is potentially dangerous though: the circuit may draw up to 20A, but the switch will be overloaded at that point which may cause it to overheat/melt/start a fire.
  • Really this is the same as wiring: you can use larger wires (with higher current ratings) but not smaller wires
• A 20A outlet is not allowed on a 15A circuit. It's not really "dangerous" since the breaker will (should) trip at 15A anyway, but it is bad to knowingly install it this way.

No.

For a receptacle to be rated for 15A, it must be able to handle 15A at either outlet. For example. If you plug one item that draws 15A into a receptacle, the receptacle must be able to handle that current.

Receptacles are actually designed to handle 150% over their rated current (or something like that, I'd have to find the specification to be sure). A 15A receptacle should be able to handle 22.5A, so your breaker should trip before the receptacle fails.

Consider this image...

Assuming the tab between the hot terminals is removed, and the red and black wires are each connected to separate 15A breakers. This is a valid configuration, and would allow 15A to be drawn at each outlet.

However, It's usually not a good idea to tempt fate, or stress test electrical components. So in theory, it will not fail. In practice, it should not fail.