Ask HN: Big bang refactor from 1M lines of php

HN,

I've read, including on HN, that "big bang" refactors are never a good idea. However I'm talking to a client who has 1 million lines of awful PHP code to do what I think could be reduced to 100k or fewer in python/Django.

I've built large codebases before and refactored chunks of them, but nothing at this scale. A piece by piece refactor to me sounds very tedious, considering the application is not terribly complex.

Thoughts / suggestions?