As far as I can tell, the only car on fleet in Group D is the Altima 2 door (unless someone can find the last of the Galants), and they only have a few of them at LAX, so you are pretty likely to get upgraded into something else anyway.
You can usually get a much "better" deal by negotiating an upgrade on site with the counter rep, as they are incented to sell upgrades and they can base what they offer availability at the time. If I were you, I would leave your reservation as it is, and see what they offer you on arrival.