Be aware that R-134A is being discontinued, though will be available for years via recovery. PAG oil absorbs moisture, so don't leave anything exposed long once you start. Read up on PAO 68 oil. No need to change the expansion valve, other than peace of mind. It adjusts to regulate evaporator temperature, not pressure, so "should work" for any refrigerant. Many older systems (GM?) even used a fixed orifice. Unless the compressor failed, not essential to solvent flush the condenser, but at least blow shop air thru it to get out most of the old oil.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans