GitHub has done this, when rewriting their permissions code. They gave a talk about it at rubyconf 2014 and you can learn more about their experience and wrapped up the ruby code they used in a gem available at https://github.com/github/scientist

You might find it useful to read their source, the same techniques could probably be easily adapted for your usage.

Unfortunately I am heading off to work and don't have time to elaborate further but to very briefly answer your question, yes I have found differential testing to be hugely beneficial. I don't use it to replace unit tests but lean on it heavily when doing a rip and replace of a sub-system such as when I am rewriting a ruby service in scala.