Can I decrease voltage to get higher amperage?
Assuming a buck converter that's 100% efficient, yes, you will be able to draw 1A from the output.

In practice, buck converters can be very efficient, often over 90%, so you'll still be able to get around 900mA output, which is rather better than the 500mA you'd get with a linear regulator.

An important part of the buck converter system is its input filter capacitor, which provides pulses of the high output current to the converter, while drawing the continuous average lower current from the input.