What will happen, likely, is that you will get to
Alamo and -- depending on the counter agent -- you will get a sales pitch to upgrade at "only the fantastically low additional cost of XXX per day." It is not a new booking.
And, considering you will be getting a "tour price" on the basic car, I have found that counter agents usually do not attempt to upsell you when you have a "tour reservation." So, you may not get the pitch. Whenever I reserve through Priceline name your own price, it comes across as "tour," and I never seem to get the pitches.
And, normally, that ptich is not a good deal, anyway. But, some counter agents do not attempt to sell you upgrades. So, you never know.
So, if the counter agent does not attempt to upsell you, you would have to ask them yourself if there is any possibility of getting whatever sort of vehicle you want. And, of course there is, but odds are the price will be even more fantastically better than if they had suggested it -- NOT!
In other words, if you ask for it, they know they have you hooked and may try to shoot for the moon and ask for a higher per day increase -- or not. Just know that when you upgrade, the counter agent gets a percentage of that extra cost, so they will usually push hard for the extra cost.
Any chance of contacting your tour company in advance and discussing it with them beforehand, rather than leaving to chance at the rental counter....
And, whatever you do, always be nice to the counter agent. No matter what. They can really screw with you and it can be very disheartening.
Over 40 some years I have rented cars, I have always went out of my way to be nice to the counter people, nice to the lot people and nice to the exit folks.
It has paid off many times in reduced rates, upgrades and "we'll just pretend that little dent was there before."
