You don't "have" to use the different switch unless you choose to. The switch is the high pressure cut-off switch and the one they have you put in is actually a "higher" pressure high pressure switch. R134a runs at higher pressures than the old R12 and therefore they recommend changing the switch out to allow the system to run up to a higher pressure before cycling the compressor off. It's basically a safety switch to prevent your AC system from grenading itself if your pressures were to go too high. IMO you'll protect your existing 15 to 20 yr. old parts better with the stock switch. I did the R134 retro on an '89 525i and did not change the switch and the AC system had no problems keeping the car cool and did not, to my knowledge, ever trip the stock HP switch. Just my .02.
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