https://www.youtube.com/watch?v=FzlYKeNcZ4M
I've said it before, but the Milo engine's backwards compatibility is legendary. This was mostly done via just endian swapping the GH2 stuff over to big endian, then the engine can just handle this old content seemingly perfectly. RB3 dropped support for certain kinds of assets, so those have to removed, but other than that, it all mostly just works.
It is interesting that they almost never really used the backwards compatibility officially and it was almost exclusively for development purposes. The only times I can really think of it being used officially was for DLC in the Dance Central and Rock Band games.








