At a minimum, if the system is now working or just low on R-12, have the system emptied by someone with the right equipment. If it empty don't worry about it.
Remove the center valve gut, just like a valve stem on the tires, and install the conversion fittings for the R-134a connections. Then evacuate (pull a vacuum) on the system, add a touch of oil if it was empty to start with, and fill it with R-134a.
The R-134a molecule is smaller then the R12 molecule, so it can leak out of any thing designed to seal R-12, such as gaskets, seals, o-rings, and hoses. But it has been found the oil and coating that form inside the A/C system seal the components quite well.
If you open the system, use seals etc that are compatible with the R134a and you will have no problem.
Flushing the system is fine if you don't get carried away, and is in fact a good thing. But if you use too good of a solvent when flushing, you will remove the old coating and you could have leakage problems.