I looked into it when the a/c on my '89 Dodge went out. I was told that you have to purge your system of all R-19 before you replace it with the R-134a. My advice to you is to let a professional do it. You may have to replace the filter also. If your system is still charged with the R-19, the old refrigerant is supposed to be collected by Federal law for recycling. There is supposed to be a stiff fine for just letting it vent out. I know, it's only illegal if you get caught. The R-134a is supposed to work just as the old does, even in converted systems. The good thing is that the R-19 is getting scarcer to get in the U.S. and therefore more expensive, so the conversion may will money in the long run if you need service later. You can still get it cheaper in Canada. I believe they still sell the R-19 cans for home recharging there. There is conversion kits available everywhere for about $45 to do it yourself. Whether you do it yourself or have a pro do it is entirely up to you.