You don't have to use higher octane. It's just you have less O2 in the air so you need less fuel to run. You will have a little less power and need less fuel. On my Suzuki Swift it makes i think about 4-5%.
It's like the same with a Turbo. A turbo compresses air, so you got more air and need more fuel. And as a side effect you have also more power
. I know someone with a Swift with a turbo went from 40 MPG to 25MPG.
You may need higher octane when you change ignition timing.