Partial replacement is a bad idea. Frequently, old steel pipes simply cannot be removed without damaging the connected pieces, making total replacment of
that branch necessary, but unplanned for. Frequently, partial
replacement makes the clogging problem worse, due to rust and crud getting dislodged and moving into remaining pipes. Leaving steel pipes that old in place
subjects you to middle-of-the-night leaks and sudden bad clogging, requiring unplanned for emergency repairs.
There is no way to fix rusty old steel pipes short of replacement.