The key to unsweating valves is to remove all of the water from the area around the valve, and prevent any trickles into the area. You will want to open any faucets possible to admit air and let water drain. You might have to blow air through the pipes or use a vacuum to suck the last water out at a faucet. It may help if you can take the bonnet off the valve.
Ferrule type valves are often longer than the sweat type valves. Check the length to see if you can cut at least one of the lines off flush with the valve body to avoid unsoldering.
If you are using ferrule type valves to get the union effect, I have seen a sweat valve by Watts, sold at some HDs, that have union type couplings soldered onto the pipe and nuts to connect the valve. They cost about the same as soldered valves.