I would recommend the following:
1. Change to a R134a compatible compressor
2. Disconnect every line and change o-rings to R134 compatible o-rings.
3. Flush the lines, evaporator, and condenser
4. Install a new dryer & expansion valve (if system uses expansion valve).
The R134a oil is not compatible with the R12 oil. It is my understanding the 134a will destroy the R12 seals if they are not changed. Also a R134a compressor is more efficent at compressing R134a.
I converted my 88 Saab 900SPG to R134a a year ago and it blows ice cold. My 87 300SDL is still blowing cold so I haven't converted a MB yet.