You just need the updated lens.
The rear light assembly is made up of 2 main parts. The main assembly holds the wires, connectors, and bulbs. The lens is placed on top of this assembly and held on with 5 or 6 nuts.
If you open up your rear light assembly as if you were to replace a burnt out bulb, you will see the nuts holding the lens on. Just remove the nuts and the lense will come off. It may be in pretty tight due to the age of your car and the fact that there is a water tight seal. But just work at it carefully and the lense will come right off.
Hope this helps.