Not many AC's made before the 1990s or so are good energy wise.That is one of the reasons there is a push to get people to turn in/replace their units from the 1970's and 1980's with newer modern "Energy Star" units.
My Friedrich wall unit was made in 1990, but has about the same energy rating as models sold today (9.2), so there would'nt be much energy savings to pay for a new unit. Keep in mind also besides changing the cooling system in small ways, the largest energy savings from new units, comes from cycling the fan with with the compressor.
Besides, one just has to love the "ahhhh" factor from the ice cold air Freon cooling units produce. Sort of like walking down the frozen foods section! *LOL*