When converting from metres to inches, for example, what is different? What is the same?
The size of the unit measure may change but the amount being measured doesn't.
I think I'm reading the question right by thinking the question is asking:
When we convert from one unit of measure to another, what is different about using that new unit of measure? And what is the same?
And so, for clarity sake, that is what I'm answering!
When we measure something, let's say the speed of a car, two standard measures is miles/hour and kilometres/hour.
What is different in using these different measures? The size of the stick we're using to measure off distance. In this case, the kilometre stick is about 0.62 the length of the mile stick.
What is the same? The speed of the car, the rate at which those little white lines flash by at highway speeds.
The same is true when converting from litres to gallons - the size of the unit changes but the amount of gas my car needs to drive to and fro doesn't change.