Check this forum out for some good info on Auto AC repair. https://www.autoacforum.com/index.php?sid=3e0261046a3035b1a082ef8d8e1a1399
I found it back in 2006 when I was going to try and fix the AC system in the 92 Cavalier my fiancé was driving. Shop wanted around $1k to fix it which was a no go. She said she would buy all of the AC tools if I would fix it myself, so I went on this forum and asked a lot of questions as well as read a lot of stuff on it, bought a few books, and did the repair. It turned out good and lasted quite a few years before the replacement compressor developed a leak and leaked out the charge.
I did convert this car to R134a at the time because I hadn't yet got my 609 certificate so that I could buy R12. I believe I used 80% of the R12 capacity of R134. I did not use a different high pressure switch than was what already was on the car.
Since that time I have probably repaired around 15 vehicles. I ended up buying a refrigerant scale for the big tanks and buying several R134a tanks and one barely used 50lb R12 tank.
I still use R12 in the 92 Cavalier that I still drive and it gets ice cold. I did finally have to replace the lines going to the compressor because they got tweaked where they bolted to the compressor and wouldn't seal up.