If your current system doesnt leak, then you wont have any leaks with the 134a.
If you are using R12 now, you do need to evacuate the system of the R12 oil. And replace it with the PAG oil for the 134a.
A lot of people dont do this, and it can cause trouble down the line. If you dont remove the old oil, and just add new oil, you now have twice as much oil... not a good thing.
You should be able to get a conversion kit (1) container of PAG oil, (3) cans 135a, a set of conversion adapters, and the can tap for under $50 at most parts stores.
Or you can go on line and get your 906 certification for $15 and buy R12. (about $40 per pound over the counter)