Bad news I'm afraid. Our supplier has just replied as follows:

We cannot swap names on rentals it will be a cancellation and re book, and they certainly wont be able to do this locally, everything has to be in the lead drivers name, unless the client has a debit card as they can use one of these at Orlando
If you do have issues at the desk, if you have internet access, just do a new booking on our website in the correct name and pay in full and you will get an immediate voucher to use at the desk (you can show them the Alamo confirmation number on your phone as this is all they need).

Then email us asking for a cancel and full refund on the existing booking as you could not use it.

Looking at the flight number you gave on the order, you're already in the air on the way to Florida, but if not, email us marked urgent and we'll get things swapped over before you depart.