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.