Wanted to pass along the experience I just had with Hertz. I have 16 rentals this year, but hadn't been upgrade to 5* which based on their site and this forum I qualified for. I called the number OKC number (405-775-3180) and had a callback (nice feature for not sitting on hold) in less than 10 minutes.
Was told by the customer service agent that Hertz only does a sweep for upgrades a few times a year, but that by calling you can ask for the upgrade. Gave my name and Hertz number and was upgraded within 30 seconds. Not sure what success I'll have the the upgrades but the process to get the 5* on my account was very easy. Good work Hertz.