Both sensors screw into the drier. I think they cost about$20-$30 each. Your low is probably working since the compressor engaged, but I have seen some of them leak next to the output wires. The sensors can only be tested when the system is working. To be on the safe side you should change both. The two sensors come with the o-rings. Then you simply screw them into the new drier and install the whole thing (need two o-rings for the two pipes).
Unless you have difficulties getting R-12 I don't recommend switching. I know that some people might disagree with me, but R-134 can never perform as well as R-12 in the old systems. This is especially noticeable in hot areas. What's the temperature like over there? BTW when you buy the expansion valve most of them are set for 134. If you decide to use R-12 your tech needs to adjust it.