only for 1.0.0 and 3.0.0 or between 1.0.0 and 3.0.0?
It will work on all firmwares but...
1. You need a NAND backup from your switch of the firmware you are trying to downgrade to.
2. You need to have used this method to update in order to keep your fuses from burning out to be able downgrade back down. For instance, my fuses are burnt up to 3.0 so I can't downgrade back down to 1.0, not to mention I don't have a NAND backup of my switch from 1.0. But if I use this method I could update to the current firmware and then be able to downgrade back down to 3.0 because I didn't burn any more of my fuses when updating.
3. Its most useful for 1.0 - 3.0 firmware users as it allows them to update without burning out the fuses that keep you from downgrading back down. 1.0 - 3.0 firmwares should be getting a unteathered softmod.
People who is above 4.1.0 can update right? Cause we are pretty much out of luck right now....
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Well, it would work if you ever wanted to be able downgrade back down to 4.1.0 but that would be kinda pointless as there is nothing special that about that firmware. Up to you really.
EDIT: "Deja Vu has been shown to work up to 4.1" as confirmed by Draxzelex via SciresM below. So I would wait to update till this is released and make a NAND backup of our 4.1.0 just in case.
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EmuNAND is still a better option as it lets you run backups separately from SysNAND mitigating the chance for bans, but I guess this could be useful to tide people over until it is released.
Agreed, but this can be useful for temporally being able to update to obtain game updates and play newer games until all of that is ready. I deleted my game updates without realizing it and can't play my carts right now so this is very useful for me atm XD.