You can "convert" an R-12 system to R-134a by just evacuating the R-12 + oil out, refilling with R-134a + oil in the correct amount, and then popping on R-134a retrofit fittings.
But that only works on some systems that are in really good shape, and that have a compressor, condenser, and evaporator that do okay with R-134a.
Even so, it's better to replace the old hoses with new barrier ones, not just because they do better at preventing the smaller R-134a particles from getting out, but in replacing the hoses, you can replace the rubber o-rings with the green ones. The o-rings are the most common leak point, so if you just recharge a system, whether it's with R-12 or R-134a, you might wind up having it all leak out over time.