Thanks to both of you for your information. This is very strange, as I can see the the 5 Star benefit mentioned under My Account > Explore My Benefits, but there's no link anywhere to enable me to re-enroll. I called IHG, they said call Hertz, I called Hertz and spoke to someone who had no information about the upgrade perk. I'll probably try one more time, but I'm just really baffled as to what happened.

UPDATE: Spoke with someone at IHG who understood the problem. He could see my Hertz information and indicated their records show that I'm signed up for the 5 Star Upgrade. That may explain why I don't see the link on the IHG website to register, as I'm already registered. He's going to bump this up to someone from IHG who deals with Hertz, and we'll see what happens. Fingers crossed.