Not really a big deal replacing them as long as you already have evacuated the system beforehand. Simply unbolt the fittings, remove the old seals, install the new seals after coating them with a bit of a/c lubricant, and then refasten them. While you are at it you might want to replace the seals under the 'VALVE, COOLER COMPRESSOR SUCTION SERVICE' that sits on top of the compressor, and to which the hose fittings are attached to. There are about 4 seals under there if I recall correctly. And then you might as well change the seal in the hi and low pressure fittings in the front trunk, or just change the valves over to R134 ones. And of course the drier receiver should be changed out at the same time. Oh, and there are also seals where those hoses connect to the long 'suction/receiver' pipes that run from rear to front of the car.
Probably best off if you go ahead and install R134 retrofit seals as they are a bit thicker than the original R12 seals, especially if you are going to use R134 or equivalent afterwards. R134 is a smaller molecule gas, and why thicker o-rings are supposedly used.