How come that in the Wii U, a firmware update cannot be reversed? What exactly has changed for it to not be possible even with homebrew?
Half-knowledge (possibly incomplete or even
wrong) ahead. To the best of my knowledge:
I know of no particular downgrade protection like the efuse thing on the Switch. So I would expect this:
I was able to downgrade from fw 5.5.5 to 5.5.1 by flashing both SLC and MLC via a hardmod to repair a bricked console. I highly don't recommend that anyone try to do this just to downgrade your firmware version. So technically it is possible, just not easy to do.
to work.
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What we are missing on the Wii U, compared to the 3DS, is a possibility to load software
instead of the normal operating system – like GodMode9. We would need an end-user ready implementation of a boot-time exploit (equivalent on the 3DS is B9S). Even the cold-boot solutions (CBHC, Tiramisu) rely on exploiting something on the main OS. If we damage the main OS, there is no way of booting into any kind of homebrew software again – for trying to fix the damage. Recovering from software bricks is way more important application than downgrading because
"I want to downgrade".
Overwriting the NAND while the main OS is active doesn't sound safe to me either (again the warning:
half-knowledge… or even less). For updates the Wii U boots into some kind of special mode at some point (can't quite remember, long ago that I updated a Wii U). On the other side, overwriting the vWii bank (" SLCCMPT") of the SLC NAND from Wii U mode is possible to repair a bad vWii mode.
Having all that said, there is no reason to downgrade either since homebrew/CFW related works perfectly on latest version. I personally advise against arbitrary experiments with the Wii U because of the missing safety net or brick protection – there is no way to recover without advanced soldering skills. I lost track of how often I fully bricked my testing O3DSXL (
poor thing…) – and recovering even from the most extreme nonsense just means waiting a few minutes for GodMode9 to restore a good backup.
Come to think about it, the 3DS lets you flash about any firmware to it, even lower versions (though too low versions won't be compatible with B9S).
B9S uses a boot-time exploit (Sighax) and provides bootloader functionality; fully independent from the main OS. Luma3DS has limited functionality on very low versions, but works fine with 1.0.0-0 (just almost no games will start with the earliest version).
