I have an old galvanized water supply system, at least 40+ years old. The pipes are holding up fairly well for now.
I was thinking about replacing an angle valve, but I do not know what to do if the threads were completely corroded or the pipe was broken off. In such emergencies what can you do with old galvanized pipe?
I am not in a position to replace the whole system with new copper pipe.
The only thing you really can do is try to unthread the nipple. You will have to hold back on the fitting it's threaded into and probably use a decent sized wrench. Heat will help, if it won't turn you might be able to cut it out. If you wreck the threads you need to try and take apart the next joint. Wreck those threads and you need to go back further and so on.
If you get the nipple out use another galvanized nipple. If you have to go back a ways use a die-electric fitting and convert to copper. Do not attach the copper right to the galvanized.