I've done R-134 conversions, and have a friend who has a side business doing R-134 conversions.

Get one of those R-134 kits from Walmart or the like. Cost $30-40 bucks.

After you replace the crunched condenser and drier, suck it down with a vacuum pump. Then, install the 134 according to the kit instructions.

You're done.

Now, if you're willing to pull the compressor to drain it, I've gotten better results from draining and refilling the compressor with a better oil than just converting and leaving the old oil in there. Noise was appreciably lessened, and engine drag seemed reduced as well. No change in cooling capabilities.