I've actually been searching this a while and 20 minutes today with the correct keywords finally brought up something valid.
This method has been known for a long time (since the King Kong Exploit days) and does indeed 'work'. Basically you swap the Keyvault (or 'KV' in short, it stores stuff like console certificates, per-box private keys, DVD key, etc) inside the NAND with the KV from an unbanned console. While technically very different you could somewhat compare it to swapping eeprom data during the Xbox1 days.
However there's a (really) big 'IF' (besides the required tech knowledge) … you need the (unique) CPU key of your banned console – without it you can't correctly write the new KV data in the NAND bin of your banned console.
Now you wonder how to get the CPU-key … well by running XeLL of course!
To run XeLL u need to perform the JTAG hack (or the King Kong exploit, but that's even more outdated) … and if you got banned the last few weeks it means you updated to the latest dashboard/kernel (else you can't get on LIVE).
And that's the problem… Microsoft patched the JTAG hack since kernel 849x (July 2009) … so no way to boot up XeLL (= no realistic way for you to get the CPU-key atm) and thus no way to swap the KV data in your NAND image.
And then there's also added KV protection (hashing) on newer motherboards (Falcon+ ?), but if you can't even get your CPU-key it doesn't matter much to do deeper into this problem.
The only way it would work is if you retrieved the CPU-key of your old banned 360 before you updated to anything over 849x. If you did that you probably know about all of this and I'm not telling you anything new
Downgrading kernel is not an option either, older kernels won't boot as both kernel and CB fuses were burned during the various updates MS performed.
Hope that explains the situation a bit (tried to make it not too technical)
On a side-note, it would probably be pretty easy for MS to detect KV-swaps (like HW-mismatches etc).
I also found this post touching base on the ultimate question from this same forum. It would make sense to have to lift the NAND and the EEPROM but as I read before, swapping NANDs themselves and the box becomes incapacitated. An interesting yet annoying bit.
Edited by Androxilogin, 01 August 2012 - 02:11 PM.