It has been pointed out that, while replacing the existing the existing water system is infeasible, building a new, downsized system for drinking water, with the existing option being used for non-potable water, may be an option:

https://www.strongtowns.org/journal/2016/2/7/chipping-flint

Hong Kong, for example, has separate water infrastructure for salt water, which is used for flushing toilets and other non-drinking uses.