I think I recall another thread saying that National/Alamo/Enterprise uses a relatively lean fleet in part because unlike some competitors, they count expected returns as available inventor. Avis and Hertz have been described here as more easily doing swaps due to customer preference. This might be why.

It might be ok if you swap an SUV or midsize for another in its class, but if not, when you get back, the next customer might be screwed.