I actually managed to downgrade my own xbox 360 by swapping the CPU with unburnt "Xenon CPU with all Zeros" I ran dev 67xx firmware on it, and I actually J-tagged it and it hangs if you want to run a retail dashboard, I believe the reason is Xebuild doesnt patch the first CPU line if you select the J-tag option, However, you can run a retail RGH image using it.
I have to build my own patches if I want to run retail on it.
Long story short I can run
Dev dashboard ( any dashboard ) on a retail console WITH NO wires or RGH ; however requires you to swap the CPU
J-tag it if you want to run retail BUT you need to patch Xebuild to by pass CPU fuse line 01 (the line with F0F0F0F ) to pass DEV CPU check.