The conventional wisdom is that rewrites usually fail:

  https://en.wikipedia.org/wiki/The_Mythical_Man-Month#The_second-system_effect

  http://www.joelonsoftware.com/articles/fog0000000069.html

This article has some specifics on the rewrite of healthcare.gov:

http://www.fastcodesign.com/3047856/innovation-by-design/mee...