You can use the old r12 hoses but the 134a will bleed though them. Not that big of deal, you will lose probably about a can of freon a year. I would definitly flush the system. I use denatured alcohol instead of a/c flush. Its cheaper and can be bought easily at your local harware store, lowes, home depot, ect. Just remember that it is very flammable. Most certainly replace the accumulator/drier and the orfice tube. And this is a good time to replace the seals in the system. R12 systems used mineral oil in them which is not compatable with r134a. Need to change the oil in the compressor. If you dont get all the mineral oil out of the compressor, no big deal really. A little left wont hurt anything. PAG oil is what they use in new 134a systems but the ester oil is a good choice for the conversion. If you do flush the system make sure you pull a long deep vacuum on the system and its not a bad idea to pour about 8 ounces of ester oil in the accumulator before you put it on as you will have removed all the oil in the system. Its also a good time to remove the blower motor and the blower motor resistor to see how much crap has accumulated in the evaporator coil. You will be suprised what you find and how much better the a/c will work with a clean evap core. You can blow it out with air or wash it with water and maybe coil cleaner if its really nasty. And of course make sure the condenser coil is clean also, escpecially between the coil and the radiator where trash collects that you cant see. The pressure switches will be ok. 134a does operate at different pressures than 12 did but it shouldnt affect the pressure switches. They are there for protecting the system for too high or low pressures, not there to regulate the system. Dont know how much the hoses would cost but new seals, orfice tube and accumulator are a must and are not too expensive. It all really comes down to how much you want to spend and how reliable you want the a/c to be. 134a has gotten to be pretty expensive over the last couple years and will probably get worse. Im not a big fan of leak stop in an a/c system so I wouldnt be tempted to put any in. Very rarely does it stop a leak and sometimes it stops up what it shouldnt. When you replace the orfice tube pay attention to what kinda garbage is on the screen. That can be a indicator on what kind of condition the compressor is in. Its not really a bad job. Just takes a little time. Pay attention to the conversion fittings, some of them require you to remove the existing shrader valve in the r12 fitting. 134a is about 10% less efficient than 12 was but you probably wont notice it. If you do it will be at a red light setting still. Most all of the conversions where pretty straight forward, some of the gms had refridgerant valves in the compressor that had to be changed but I cant recall every seeing anything on a RBV like that.
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. Anyways really and truthfully if it was just the relay I would flush the system and remove all R-12, place all orings since its down anyways (good practice) and install a new orifice tube and accumulator. Both arent really expensive but easily replaced. Also take your compressor off and turn it a couple few times to remove and old oil, then put a couple ounces of ESTER oil in compressor (available at Advance or pretty much any auto store, I got the kind with UV dye) as well as an ounce in the accumulator. Then vacuum it down and check for leaks. If all is good fill with R-134a to about 80% of R-12 volume and enjoy. 
