[velosol]

Changing properties of electricity (ie current to voltage or vice versa) requires a trip through the magnetic field and that means varying the current through a conductor. With AC that's easy as it's already changing and so you can trivially (passively) convert between voltages at the same frequency with the efficiency cost of heating the conductors of the coils used in the transformer.

With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another.

I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).