Replacing Angle Stops
The plumbing term for shut-offs under sinks and for toilets is angle stop. The majority of shut-offs out there are "Compression Angle Stop." These are the shut-offs to your faucets and toilets. Having a "Compression Sleeve and Nut Puller" will simplify this task. This tool allows you to take the nut and sleeve off in one action. If the nut and compression sleeve are difficult to remove even with this tool, you may have to cut them off. This is not a problem if there is plenty of pipe sticking out of the wall. if there is, cut the stops off. Life will be easier.
Now comes the tricky part. If you have removed the nut and sleeve and the pipe is compressed ( the new nut and sleeve should not be loose on the pipe) you will not be able to install a new compression stop on the compressed pipe, because it will leak. A remedy is, if there is sufficient pipe remaining sticking out of the wall, you can cut back to good pipe and install your new stop. If this is not the case you may be able to solder on a male adapter and install a "iron pipe" angle stop. This would eliminate having to open the wall and solder on a new stub-out. Plumbers tip: Put some "plumbers grease" on the threads of the new stop. This will help lubricate the threads and make tightening easier. If you can not for the life of you loosen the nut on the old stop and you have sufficient pipe sticking out of the wall. Cut the stop off with a tubing cutter. It will save you all kinds of frustration. I know. I have been there.