I start with a working GPL project. I wholesale gut out modules and rewrite them from scratch. Sometimes my rewrite would combine a few modules into one module. Other times my rewrite would subdivide a module into a few more modules. So I don't even preserve the modular structure. I'd do this until nothing remains of the GPL code. As I go I'd be modifying the interfaces a bit as well, so even interfaces between modules would be unrecognizable from the original.
See the strategy. I'm just using the GPL project as a test harness as I insert my rewrites. It would be kind of a dwindling test harness. In this case, where there would be no GPL code left in my project, do I have any obligations under GPL?
I found this FAQ, but it is about making mods not replacing all code.