What I can't answer is the evaporator question. The '91 and newer models use the improved unit, but I believe it is a job and a half to change.

No easy way to remove the compressor... just get under the car and figure it out. You do have to remove it to get the old oil out and to change fittings. Take advantage of the opportunity to put in new mount bushings, and new drive belts as well.

The full retrofit kit from Volvo is worth it; you get all the O-ring seals you need, plus a new drier plus new hoses. Buy cheap, buy twice. I got my official Volvo kit from FCP (but again, I have a '91). I would look at the drier connections on a '91 or newer model to see if/how they differ from your model. My guess - *only* a guess - is that they are similar. Be sure before you invest.

I suspect what you call the "muffler" is the drier. Sorta like a large tin can mounted on the firewall. If you live in a humid climate (which is anywhere outside the SW desert) you should replace the drier when you open up the system for more than 5 minutes. Refrigerants are highly hygroscopic and water vapor compromises their performance. They are a replacement unit, like the other filters on your car.

With the hoses off, flush the old components out (evap + condensor). You can get spray cans of the stuff from any auto supply store. What's helpful is a shop compressor to blow the lines clear after the flushing agent is used.

I'm not sure if your Volvos has an expansion valve or an orifice tube. If the latter, replacements are about $3 from an automotive AC supplier. I would look at AC components listed by an auto AC specialty house to be certain - a generalist like FCP is probably not the defintive source of detailed info like that.

If you do luck out and your system uses an orifice tube, I recommend the variable orifice (VOV) if you drive in stop-and-go traffic. About $25 from the AC suppliers. Even if you opt for the conventional one, put a new one in. These are effectively a trap for gunk in the refrigerant system. And look closely at the old one you pull out... if it's loaded with black crud, that's a tipoff that you probably need a new compressor (and a serious flush of the rest of the system).

If your system uses the expansion valve, you might get away with re-using the old one, but recognize that these are a common problem source even on stationary AC systems. Yours is 20 years old.

Every O-ring in every joint in the system should be replaced. These are all under the hood, and come in the retrofit kit. Lube each with just a smear of the proper system oil before installing; this will help prevent tearing. If you run short, you can get replacements at the local auto supply place but you'll pay $12.50 for an assortment that has 20 rings you'll never need.

The changeover is conceptually simple, but as with so many things the devil is in the details. Go slowly, do it right.