Ohh! Interesting. Same deal in the vast majority of the USA except that most people switch their plates anyway not knowing they can keep their old ones.
Every state is different, Doug.
In Washington (the state), the tag stays with the car; so if you see an old car, you see an old plate. Drive around for a few days, and you see a couple of different designs. There's a few exceptions, for example, you might see a new plate on a really old car because the old one was unreadable or damaged.
In Florida, you get a new plate every five years, and it stays with the owner. It is transferable from car to car (unless for some reason you go from a motorcycle to a car, or from a car to a 5-ton truck). Regardless of the condition of the plate or what car you drive after five years, you get a new tag.
They enclose a little statement telling you "this is necessary because the reflective coating wears off", but I don't buy that. The coating looks the same on a 5 year old tag as it does on a new tag, at night. The real reason is that we have a lot of prisoners, and it's best to keep them working.
The other thing I've heard is that if your car is a certain age (I think 40 years or older), you can use any tag at all on it, as long as it's from Florida. Some of the tags were really colorful, although personally, I want the pre-1980 issue tags that are as boring as can be.