Can I plug a washing machine requiring a 15A breaker into a 20A outlet?
Actually, you don't need to change anything. All 120V 20 Amp receptacles are designed to accept 120V 15 Amp and 120V 20 Amp plugs. Because of that, all 120V 15 Amp devices are normally designed to work properly in a 120V 15 Amp or 120V 20 Amp circuit. This is true for appliances like a washing machine but even for something as small as a cell phone charger that actually uses &lt; 1 Amp.

Once you get past 20 Amp, the design of the sockets change so that you can't mix and match. And you can't plug a 20 Amp device into a 15 Amp receptacle - it won't fit.

Bottom line: No problem.