R12 is not illegal.  Though it is no longer in production in most countries, Congress has mandated that R12 be available to qualified technicians in reserve & recycled forms until the year 2030.

Most people are concerned with converting to 134a, fearing that it's an inferior refrigerant.   With a few exceptions, an R12 system can be converted to 134a to function with every bit as cold of temperatures.

How do I covert my R12 system to 134a?  Simple enough.  Regardless of the type of system you are using, there are some basic rules that accommodate all 134a conversions. 

1. Change the oil in the compressor to an oil (PAG or Ester) that is compatible with 134a.  PAG oil is compatible only with 134a.  Ester oil is compatible with both R12 & 134a.

2. Replace the filter-drier, accumulator or the desiccant in the VIR ***, depending upon the type of system being converted.  This should be done regardless of whether or not the system is being converted to 134a.  Keep in mind that NOS driers will not be suitable for 134a.   ***  For models equipped with a VIR (some 1973; all 1974-76 exc. truck & some early model 1977 Corvette) it is recommended that you replace the VIR with a VIR eliminator kit for best performance.  VIR equipped cars are one of the exceptions to systems that can be converted efficiently since the VIR's calibration cannot be changed.  While the VIR will still work, it just won't be as efficient. 

3. Flush all original used components to be re-used to insure against contamination.  Though it is not necessary to clean every last drop of oil from the system, contamination is the number one cause of air-conditioning failure.  This should be done regardless of whether or not the system is being converted to 134a.  SPECIAL NOTE # 1 - Do not flush the compressor.  Simply drain the old oil and replace it with a fresh charge of oil.  If heavily contaminated or there is metal in the oil, replace the compressor or have it rebuilt.  SPECIAL NOTE # 2 (SEE PHOTOS BELOW)- The plate & fin evaporator should never be flushed in the car.  Instead, start by blowing compressed air through the evaporator to get as much of the oil out of it as possible.  If the oil is contaminated or there is caked on contamination in the ridges of the end cap on the bottom, then it will need to be removed and rotated while flushing to insure all flushing agent and contamination (loose or caked on) is removed.  If this is not done, it will be injected into the suction valve via the oil return line, often causing failure of the valve (causing to stick open) and other components.  SPECIAL NOTE # 3 - There are a lot of flushing agents on the market.  You should use something that flushes clean, leaving no residue.  We use lacquer thinner (high-grade/no mineral spirits) on our parts.
   

4. Replace all rubber hoses, using barrier rubber hose.  Though your original used hoses may hold in the short-term, they will ultimately leak or burst!  A good way to physically check for hose defects is to remove the hose and bend against the natural bow… inspect for dry-rotting, splitting, bubbles or holes at stress points.  With any of the above, replace or restore.

5. When converting an R12 system to 134a, charging amounts vary from system to system.  A good starting point for charging 134a on your freshly converted system is typically about 70% of the original R12 charge, and the final charge amount will be no more than 80%.  You may need to add or subtract a little, depending on your gauge readings & vent temperature.  Federal law mandates a sticker and charge port adapters remain on the car once converted.  The sticker usually contains a location to indicate how much 134a was charged in the system.

6. General motors vehicles using a Hot-Gas-Valve (HGV), Suction Throttling (STV) or POA valve will need to be calibrated to by-pass at 20psi.  Since this cannot be performed on POA valves while charging the system, it will need to be done prior to installation.