I can tell you what happens if you don't- absolutely nothing. I've never ever called to change the car on record and I've had the sunpass for several years and multiple trips to WDW. It's very convenient and I love flying past the cash toll booths when others have to stop.
I think the reason you are suppose to is the pass malfunctions (or the reader does) then they can match you up. (Otherwise they charge the rental car company who charges you etc..)
Found this on one of the news stations....
In the event that the transponder batteries go dead, or you simply forget to put the transponder in your car, the correct license plate information will help us identify you as a valued SunPass or E-PASS customer.
This is important because the name of a SunPass or E-PASS account holder may not be the same name listed by the Department of Highway Safety and Motor Vehicles as the owner of one or more of the vehicles authorized to use the transponder.
Why is it so important to notify SunPass or E-PASS when your vehicle license plate changes? If the signal from your transponder cannot be read and your license plates have not been registered with us, we will not be able to match the vehicle to your account and you may receive an unpaid toll notice in the mail.
If you receive an Unpaid Toll Notice, a Violation Warning or a Uniform Traffic Citation in the mail, please call SunPass or E-PASS immediately.
If you fail to contact us by the date on the Unpaid Toll Notice or Toll Warning Notice and do not bring your account current, you will receive by mail a Uniform Traffic Citation at the address on file with the Department of Highway Safety and Motor Vehicles, in compliance with Florida statute.
You have 30 days to repond to to a Uniform Traffic Citation. If you have not contacted SunPass or E-PASS, the citation will be filed with the Clerk of the Court.
The problem is that it generally takes the rental car companies longer then 30 days to notify you that your "pass" didn't read. (Plus you still have to pay them the admin fee!)
After reading the article I referenced below I do have one piece of advice for you SunPass users. Log on periodically and check that accout!
http://www.tampabays10.com/news/local/story.aspx?storyid=59806