I Have a saddle valve that allows water flow to a furnace humidifier. I can not shut it off nor turn the valve stem. Can a saddle valve be replaced or does one install a new piece of copper pipe and then install the valve?

Shut of your main and open a faucet some where to blow off pressure. Remove the two screws on each side of the saddle and it should come right off with a just a little rocking. Chances are good the original saddle pierced its own hole when it was installed. To match the old hole with the new piercing point close the valve almost all the way when you try to install, this will help you align the pin with the old hole.

Your suggestion confirmed my thinking how to replace an old saddle valve with a new one. It worked perfectly once I aligned as you suggested. Thanks.

Your suggestion confirmed my thinking how to replace an old saddle valve with a new one. It worked perfectly once I aligned as you suggested. Thanks.