From what I researched all you need to do is change the service ports. Here is what I have gathered: evacuate the system and change out all the o-rings. You will also have to add some PAG/POE oil to the system, then add the r134a and that's it. Now there will be many people that tell you to do a flush or that you need to buy new components because r-12 and r134 don't mix. It's a myth. You can mix r-12 and r134 and not have any problems. The molecular make of r134 doesn't allow it to bind with the mineral oil used by r-12 so if you don't add the PAG/POE oil then the mineral oil will just sit at the bottom of your compressor and not be carried through your system to lubricate everything. You will most likely have several members chiming in to either agree or disagree with what I just said, but if you have r12 in your system now then do this: evacuate a little bit out into a container that has r134 and see for yourself. Also mix a bit of mineral oil with PAG/POE oil and see for yourself. I still have the bottle of mixed oils with nothing that was of ill effect. I will caution you this: use protective gear and use a container that is vacuumed. I did this a while back before I converted my system and I mixed the r12 and r134 with both oils and the container began to build pressure from the gases expanding. I was able to reattach my vacuum pump and just cycle the gas. Hope this helps.

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1990 190E 3.0L