Just one correction on this…
If you find that your 20A circuit has 14 gauge wire, CHANGE THE BREAKER TO A 15A. The receptacle really doesn’t matter. The only reason to change the receptacle at that point, is to dissuade someone from thinking that a device that requires a 20A circuit can be supported on the line - and if they did plug it in it would pop the circuit breaker (which is now 15A). So the receptacle is really just to help you avoid popping the circuit breakers (and relying on them).
People get confused about this all the time…the breaker is a safety measure that trips when the draw in amperage on the line exceeds the rating of the breaker. It doesn’t modify the power going through the lines - the same amount of power goes to your devices regardless of the amperage of the breaker (at a given voltage). Your device is the thing that determines the amperage draw. Your device is going to draw a certain number of Amps regardless of the circuit it’s on. The wire on the run acts like a resistor, and will heat up as electricity goes through it. The thicker the wire, the less it will heat…so thin wires heat faster. The wires are rated as to how hot they can get before insulation start to disintegrate (and they get hot enough to start a fire).
So…the whole 15A vs. 20A breaker thing is to protect the wires. The weakest link in the line is going to be the lowest amperage rating of wire (or receptacle, if you’re chaining through the receptacles) on the run. You HAVE to choose the breaker based on the lowest amp rating on the line - or you run the risk that the line will take a higher load than the wire can handle, the breaker will not trip (because it is a higher rating than the weak link), and the weak link can heat and potentially, over time, burn.
But there is no hazzard in plugging a 20A device into a 15A circuit - it will just trip the circuit breaker