I have reverse converted several cars and it's not that bad. You will need; flushing agent, oil, R12, o-rings and a filter/drier along with, of course basic a/c service equipment and compressed air.

Break all connections and put flushing agent in each component and blow out thoroughly. If you don't have a flush cannister you can use brake cleaner. Thoroughly flush all components with the solvent EXCEPT the compressor. Flush the compressor with oil. Once everything is flushed and THE FLUSHING AGENT THOROUGHLY BLOWN OUT, put everything back together putting in the new filter/drier last. When putting it together, distribute 8 oz. of oil in different components. Turn the compressor a couple of dozen turns by hand to make sure it's not liquid locked.

Make sure the filter drier does not go on until you are ready to evacuate as soon as it's cinched into place. At that point, evacuate and charge with R12 normally.

The 123 and especially the 124 MB's are some of the poorest candidates there are for conversion to 134. A reverse conversion would be a very good step.

Good luck