Briefly: replacing only some of the pipes can cause lead levels to increase due to (1) disturbance of nearby lead pipes during replacement; (2) ongoing galvanic effects near the replacement; (3) interfaces between the utility's new lead-free lines and a customer's old lead lines, if the customer does not choose to replace at the same time the utility does.
Point (3) is especially troublesome. Even if the utility replaces all its lead pipes, things could get much worse for customers that don't follow suit right away.
For (much) more: http://investigativereportingworkshop.org/investigations/tox...
All these issues are just about pipe replacement, as distinct from higher lead levels due to pH changes or use of chloramines rather than chlorine (which was a surprise, see http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1817707/).