From www.carseat.org -
"Expiration date - In the last few years, CR manufacturers have been putting "expiration dates" on their products. This was begun in response to infant deaths from air bags. The companies realized that they had millions of products in use that said nothing about the danger to a rear-facing child. The concern became one that "best practice" and regulations change over time, so that a child restraint becomes "obsolete" and less effective than a new product, much the same way that medicines may change or become less effective after some time has passed. A current example would be with tethers--older CRs do not have them, and this is potentially a situation where a newer product will be more effective than an older one.
Taken at face value, the expiration interval (ranging from 5 to 8 years, depending on the manufacturer) is from the date of manufacture, which is what governs the labeling, certification, and other rules that apply. Whether to continue to use an "expired" CR is a judgment call, depending on the alternatives. It will work as well as before, but it is important to know what regulations and warnings may have changed and the risks involved. It is also important to take the expiration date into account when purchasing a CR that may have been manufactured several months or even a year or more before."
Hmmm, sounds like the main "upgrade" in newer car seats are stickers warning about air bags. I would check for any recalls, check the condition of the seat, and use it if you are satisfied.