Hi, just did it to R134a. You need a new expansion valve for R134a (4B0 820 177, 15€) and a modification kit (2 valves, 25€). That's all. There's a little risc that your A/C will be leaking because R134a is much thinner than R12. Maybe you have to refill it every 2 years or change all sealing rings. But that's still cheaper than the complete modification as Audi recommends it. Technically there's no problem according to my A/C specialist. R413 would be the correct choice, but it's not available here because almost nobody needs it.
Change over to the Propan / Butan mixture known as Envirosafe ( and other mixtures of the same ) . I am using it in my 87 GTI (sold), our 81 XJ6, our sons SVX and in my 93 V8. You shold change over to R124 fittings and add a sticker to the car. One of the advantages is, that you don't even have to evacuate the system. ( I do evacuate mainly to check for leaks.) The system pressure is lower and the mixture cools more than R12. FYI , if I remember right Albert Einstein used this mixture as well.
The modification kit are 2 valves, one at the dryer and one at the T-connection at the right and a label. There's no need to buy it because every A/C maintenance station has it. These are no specific Audi parts. The only part you should get is the correct expansion valve.
