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After fresh water is added to a sealed system, the oxygen in the water corrodes the ferrous metals until there is no oxygen left. The water is then no longer damaging to steel until free hydrogen in the water makes the water somewhat acid. This means that a small sample of the water in the system should be tested every few years to see if the water is acid and needs a little neutralizing solution added.
This also means that every time you flush the system, you add oxygen to do new corrosion, so flushing regularly damages the pipe. If the water has been left sealed in the system and no new water had to be added to make up for leaks, hot water system steel pipe will last hundreds of years.
A steam system is open to air all the time, yet pipes are still there after 50 years. A water-filled system has far less corrosion.
If you will worry that the system will have a leak, then changing pipe while the walls and floor are open will ease your mind.
