I need to and am replacing the whole shut off valve assembly for the toilet. I cannot get the assembly off the copper pipe. It keeps spinning. I am now using two wrenches, one to hold the assembly in place and the other to turn the nut that holding it on the pipe. It worked until nut met the assembly and then became to tight to keep turning. So now it's stuck unless I re-tighten it. I have no idea on how to get the thing off.
Btw, I didn't just change the handle/plug units inside because it's a different style and won't happen.

Ahhh... Compression fitting. That's why it wouldn't let me take the nut off!

Ahhh. Lesson learned. You can usually leave the nut and compression ring. I use a small amount of plumbers grease on the old ring, tighten carefully so you don't crossthread. Have never had a leak.

Brain's bang on! When we swap out a angle stop we never remove the old compression nut and feerrule, We simply reuse them. Good luck, Tom