I would not approach this rental expecting that you will get an upgrade.

Of course you might. Hertz carry relatively few PCARs and even fewer LCARs, so they are less likely to be "forced" to offer you an LCAR as they have plenty of those and no PCARs. You are more likely to be "upgraded" into an SUV than a larger sedan.

The fact that you doing a one-way to CA may not make much difference, as LAS tends to have an awful lot of CA cars on site, though they will probably try to give you a car to repatriate if they can.

PCARs these days are Maxima, Buick Regal, VW CC, Ford Taurus and Hyundai Genesis. LCARs are Lacrosse, Avalon and a very few Chrysler 300s and Cadillac CTS. They don't seem to have any Lincolns on fleet.

So, I think you need to decide if you can justify the price difference to yourself, and book accordingly.