Hi,
Well, from an ecological/sunburned penguin/"we're all gunna die"/God loves trees point of view...please don't release fluorocarbons into the atmosphere. Any registered shop will have recycling equipment to safely suck the freon from your car (until their tanks get full and they have to dump it) .
All Eco-terrorist kidding aside,though not absolutely necessary, yes it is a good idea to replace the dryer when working on the system.It isn't an absolutely necessary thing, but if you replace the switches the system will be exposed to "air" longer than if they did it. It's a personal choice sort of thing. I do my own A/C work(under license by he state of Wisconsin)and I know how long I've vacuumed a system, if it has been open etc... so I don't really feel the need to replace every single dryer if the system hasn't been exposed to moisture(air) for very long.
Regards,
Randy D.