Did you rent the 911 originally from LAX?

If you did, then you will not have any problem exchanging the car either because it has a mechanical issue or you just want to try an M5 or C63, I have done that multiple times.

If you did not and only want to exchange it to try a different car it might be a bit difficult. As you said hertz is very particular to get dream cars to the original location, apparently they rent so well they might never get them back. However if you truly have a mechanical issue I am sure they will swap it for another 911 or whatever you choose.