Maximum temperature change in cast iron boiler to avoid thermal shock?

I can't think of a reason to change the water in your system unless you want to ruin some pipes and make it leak. that black water is saturated with iron and won't cause any more damage. If you put new oxygenated and clorinated water in there, it will disolve some of the iron, and clorine is a great oxygenator. So it will do even more damage.