The shut off valve for the bathroom sink is really old and can not be turned off. The bathroom remodel will take awhile so I can't go that long without water in the whole house. What is needed to replace the existing shut off valve so that we can remove the old sink and continue work on the bathroom? Is there a way to seal the pipe?

There are three types of angle stops. Compression, threaded and soldered.
If you can tell me what you have I can tell you how to remove it and cap off the stub-out. A picture would help. Regards, Tom

How do I change a SOLDERED connection to install a compression shut off valve or do I have to solder on a shut off valve?
Thanks

If the pipe is long enough you can cut it and use a compression angle valve, other wise you have to use a tourch to un do the solder,see the images below, good luck.

John