A similar, more concrete approach is parallel implementations, as written about by John Carmack[0]. I suppose the main difference is that parallel implementation has you very explicitly and intentionally leave the "old stuff" around until you're ready to flip the switch. I've used this approach in large-scale refactorings very successfully.

One of the benefits is that you get to compare the new vs old implementation quickly and easily, and it lets you raise questions about the old implementation; every time I've done this I've found real, production-impacting bugs because the old system was exhibiting behaviors that didn't match the new system, and it turned out they weren't intentional!

[0] http://sevangelatos.com/john-carmack-on-parallel-implementat...