There seems to be a question concerning this swap which is relatively easy. First of all, the compatibility issue is a result of the difference in the oils that these refrigerants use.
Mixing mineral/ alky-benzene oils (r-12) w/ the ester based oils (r134a) will make glue! Make sure that you get all of the old oil that you can possibly get out of the system first. ( I routinely open these systems @ the service ports and flush w/ either dry nitrogen or co2., but it is not absolutely necessary as most conversion kits now contain ingregients that stabilize the slight amounts of oil that is left <3-5%)
Next, because the drier is prone to catch and hold relatively large amounts of the old oil - replace it! They are cheap, and they also keep contaminates out of the expansion orafix/strainer.
Next, chg out the orafix/strainer tube because the sizing is slightly different for the respective refrigerants and you want that air to be cold - thus volume of refrigerant metered into the evaporator coil is important (will work w/ the old one, but not nearly as efficient!)
Next, go on and pull a decent vacuum on the system as anything NOT 134a refrigerant will only serve to decrease the system'* cooling capacity (best case) but also his will remove moisture/air from the system and moisture and refrigerant oil makes acid! ( you can rent a vacuum pump )
Finally, you need to add between 2-3 ounces of the correct oil for 134a and weigh in a charge of between 82-85% of the original charge requirement by weight! The important thing here is to provide enough refrigerant in the system to raise its pressure enough to adequately condense the refrigerant into a liquid as it leaves the condenser. If you have a sight glass, then look for the bubbles to disappear as the pipe fills w/ liquid. You can also tape a temperature thermometer to the liquid line where it leaves the condenser coil and slowly add refrigerant until the pipe'* temp is between 5-10 degrees cooler than the TEMPERATURE (NOT pressure) shown on your high side gauge. (this is sub-cooling. ) If you will follow these steps, then you will have an A/C system that will freeze your buns off even on the hottest days and will rival the original equipment for reliability and service life! Whew, sry bout being long-winded, but this is the BEST way to do it!