Yes, you should replace the accumulator. This is standard practice whenever flushing and refilling the a/c system. If you're doing a conversion, a lot of people advise changing all the o-rings in the system because the old rings are "incompatible with R134a". I think I mentioned this before, an ASE-certified mechanic who works for one of my clients claims that he never changes o-rings. He showed me his 1989 Jeep where he did a conversion several years ago without changing o-rings and the a/c is still ice-cold, and high/low pressures never budged. So if you think you need to change them out, go ahead.
One of the hardest things about changing a/c parts is getting proper torque on the fittings so that the o-rings are seated. This must be done by feel, with two huge wrenches. If you've never done this before, it feels like you're going to break the connection. These parts are all aluminum, so you have to put just the right amount of leverage on it; too little and you won't seat the o-rings, too much and you'll kink the lines. You'll learn pretty fast if you've seated the o-rings after you pull your vacuum to -29, because if the vacuum doesn't hold, you haven't tightened the fittings enough.
Now, a lot of people disregard a clean condenser when doing a/c. Well, it's common practice to pull and clean the radiator fins on the C4, but the condenser fins also can become clogged with debris over time. It's very important to have proper air flow over the condenser, and when the fins are all clogged with gunk the a/c isn't working to its full potential. When I did my a/c, I bought a new condenser, but you should consider either buying a new one or cleaning the old one. It's pretty easy to remove the fan shroud and pop out the condenser, it might be a good idea to pull both the radiator and condenser and giving them both a good cleaning to get maximum air flow through the fan shroud. Your car will love you for it because it will run cooler.
Any components that you're not changing need to be flushed with solvent, you can buy it from any auto parts store, comes in a metal can. The evaporator is difficult to change, I read a tip here awhile back about removing the blower fan and looking through the hole to see the state of the evaporator fins, if they look pretty good I wouldn't bother changing the evaporator. Also, you may know that the compressor can't be flushed. Therefore, it needs to be removed and you can turn it upside-down over a small bucket and let the oil drain out, the last time I did this I let it drain for a whole day to make sure I got all the old oil out. When everything is clean, you put two ounces of the new oil in each component: compressor, condenser, evaporator, accumulator.
At this point, everything is bolted back up, you check all your connections, pull a vacuum, and keep your fingers crossed that the vacuum holds for a couple of hours. If you haven't done this before you'll need a good set of manifold pressure gauges to read your pressures.
I guess this next part goes without saying, but it's the most satisfying part of a/c work: recharging the system. You close your connections, hook up your refrigerant, bleed the line, and start charging into the low side. Here's a site that might help you:
http://www.diynet.com/diy/ab_auto_air_conditioning/article/0,2021,DIY_13674_2276708,00.html
