85 380SL A/C question

[twochaps]

My 1985 380SL A/C is not providing any cool air. It looks like the refrigerant is properly low or gone. The car has been stored for several years. I know it is almost impossible to get the old freon, so I asked a mechanic what could be done without retrofiting a whole new system with the new refrigerant. He said he only needed to vacuum out the old stuff, install a couple of new couplings and then add the new R134 or whatever it is called. Question: Can or should this be done without ruining the old A/C system?