The old refrigerant will need to be vacuumed out and disposed of correctly, that is why a qualified tech should be doing it. After that you replace all the O rings with the new ones and recharge the system.
There may be a difference in how each gas effects different seals & gaskets, so they probably should be changed to compatible items. But the cost may not be worth changing until there is a...
You don't. The oils for freon 12 and 134a are not compatable. There are substitutes for freon 12 out there. I belive on is called sure shot. There very resonably priced.
you make a chart and on top for example you have 2 gallons then at the bottom you would put gallons. and say you want to convert to quarts you put over the 1 gallon 4 quarts and it's simple cross...
pick up a bottle of HOT SHOT FREON, You wil need a EPA card to buy this. Just vacuum out your system then add about 12 ounces of the hot shot freon. HOT SHOT is a direct replacement for R12.