It's not an upgrade per se, it's just whatever vehicle they have available. If they're running low on sedans (which often happens), they'll start giving out SUV

It's not an upgrade per se, it's just whatever vehicle they have available. If they're running low on sedans (which often happens), they'll start giving out SUVs to people who'll accept them, keeping sedans in reserve who want them. NYC has some crazy rental experiences because the lots are so small. One time I got "upgraded" to a 3 row minivan (which costs 2-3X the base economy car rental price) because that's all they had left. It was not the ideal car for a road trip for my girlfriend and I, and we ended up spending more on gas, but they didn't have anything else available. Similarly I've often gotten free upgrades to convertible sports cars (e.g. a Mustang) just because they have a lot of them and they have extra on that particular day and want to reserve more boring options for picky people.