Yup, I did this every year when I was going to the states for 3-4 weeks in summer. I started booking in January, then about every week I checked the price for the same summer date, in general it was cheaper so I cancelled/rebooked, multiple times.
Also instead of rent from airport, I rent from smaller places (that handle Hertz/Enterprise/National/etc) and I always booked a subcompact, and 50% of the time they had none and gave me a compact/full size car. I had multiple Yaris yes, but one time I got a Dodge Caravan, a Fusion Sport V6, a Flex, a Jetta, a Soul, etc.
Similar happened to me, I booked a gas car but received a hybrid instead.
I'm curious, what's the advantage of renting not from the airport? I prefer the airport because it's easy to locate, especially when the airport is new and I'm dizzy after long flights