If you have an old R12 system and want to upgrade it to R134a, kits are available to do this. R12 is very expensive if you can find it. Admittedly, it works better than R134a, but given the expense and the potential environmental damage with using R12, it makes sense to retrofit your old system.
You might be surprised at what comes in a retrofit kit: Usually, it’s a couple of fittings and a can of oil. As stated earlier, it's not a good idea to mix refrigerant oils. The ideal situation is to evacuate the system and remove all the old oil before adding new oil. This will be impossible without the correct equipment. Once you've done that, you can then add the new oil in the correct amount and install the new fittings. But beware of these fittings. They often leak.
The takeaway here is that if you have an R12 system, it's a good idea to convert it to R134a; just be aware of the potential for leaks and the recommendation of changing out the refrigerant oil.