If the nut is all the way loose, the valve should just pull away with little effort.
You have some options from there
1) IF the nut threads on the new valve are the same, you can leave the nut and compression ring in place and connect the new valve. This usually works with no issue. ( The big IF is the threads)
2) Get a compression ring puller from any hardware store. This will remove the nut and ring. Now the only issue is if the original ring was overtightened, the pipe may be dimpled such that it will be difficult to make the new ring seal . See #3
3) You can solder a male adapter on the copper and use an IPS valve.