The R/D should be replaced any time the system is opened for any length of time. If you have really good vacuum pump and a somewhat new R/D, you might get by with not replacing it. In the case of a refrigerant conversion it MUST be replaced because it has accumulated the wrong type oil.

A conversion involves flushing the oil out of ALL components, replacing the R/D and distributing the correct amount of oil throughout the system; couple ounces in the condensor, a couple in the evaporator, etc. After everything is sealed up and while the vacuum pump is running, turn the compressor about a dozen times by hand to make sure there is no liquid locked in the compressor.

I've never heard of replacing sensors as a preventive maintenance step. As soon as you open the system, pull the R/D with sensors attached and seal it in a plastic bag. Then put the sensors on the new R/D just before putting it in place as the last item to seal up. Start your evacuation as soon as the R/D is in place and sealed. This pulls moisture out of the system before it has time to start soaking in the R/D dessicate.