Congratulations!

You are the first Air Conditioning question, that I have seen, for the upcoming season! A wee bit early for summer but that depends on where you are with or without the use of a calendar.

You are correct the low pressure switch is the only control for maintaining a frost free evaporator in '91-'93. I think '93 was the first year of R134a.

The switch may have gone out of calibration and you should use a low side gauge to get a temperature/ pressure reading of about ten degrees below freezing out there in the engine compartment. This is the refrigerant temperature so the coil will run right about at the freezing mark on the outside of the tubes.
I don't have a TP chart available but it should be about 25 to 30 psi, if I remember correctly. Both refrigerants, 12 and 134a are close at these temperatures even though yours was converted.

It is normal for the accumulator to sweat with water and frost a wee bit on connections once the cabin is stabilized. Frost on the rubber suction line, is not something you do not want to see, as liquid refrigerant can get back to the compressor and slug the pistons.

Replacement is a straight forward change out as there is a shut off valve below the switch that should engage if the switch is remove.
I think it is possible to readjust the switches overall set pressure but not its differential. In most cases, it has to be replace if the spread is not far enough to allow a defrost rise in temperature.

Hope this helps!

Phil