I sent an email to a contact at TI about this thread, we'll see what comes of it (maybe nothing considering this is not their area, but maybe they can forward the email to someone else). Anyway, I'll let you know.

I agree TI Care's response about basically using exploit-needing community tools is both hilarious and sad.

If you absolutely need to downgrade and are decent with Linux, you can use a program called flashrom on a Raspberry Pi (any model, really) with something called a Pomona clip (SOIC-8) attached directly to the calculator's memory chip to manually force a downgrade. I can confirm this method actually works, but I had the foresight to backup my calculator's ROM before upgrading; consequently, I never wrote the code needed to modify a ROM for downgrading (as opposed to just restoring a backup). But if you feel bold and confident enough to disassemble your calculator and don't mind buying the requisite hardware, I can privately guide you through the process.

It's technically possible to use a $4 Pi Pico instead of a full $35 Pi, but I haven't researched that and you may be on your own.

I figure this doesn't upset the détente because any student who can use engineering test equipment to hack their calculator probably doesn't need to cheat.

Well, a software way exists too, it's just not public. Probably easier for this person to be privately given the right tool to go back to 5.6.0 since that's the legitimate exam usecase...

Anyway, the TI person replied, they are now aware of the issue, so we can assume it'll be escalated.

Bad News: OS 5.6.1 not allowed for exam use. Exam Board not gonna update the thing cos TI in Singapore won't. CERMASTR 3.0 also blocked downgrade. Looking like I need to buy a new one

You can try to get in touch with critor directly (no promise, but 5.6.1 -> 5.6.0 isn't something that would help anyone cheat AFAIK...) maybe he can help you, since TI apparently isn't able to...

It really bugs me that TI makes an OS you can't downgrade, publishes it on their website without warning people that it isn't allowed in certain countries, and then doesn't even really attempt to help fix the problem.

Like apparently TI has a webpage for Singapore OS and apps only but there's no SG website for TI. Then I keep on getting redirected to US website where the entire thing started off

What if we found a way to edit the ROM in a hex editor? Maybe someone could find bytes that could be changed to allow downgrading. Though doing such a thing would be dangerously risky.

That's pretty much exactly what Bootswap does when you hit "disable OS protection" - the issue is that Bootswap doesn't work on 5.6.1, so there's no way to flash a ROM to the device without using a hardware flash programmer.

Anyways, the issue isn't that we can't write a downgrade tool (CERMASTR would work as one if not for code specifically preventing that) but that we won't, as it seems that TI has implied that they only left the arTIfiCE exploit in on the condition that we don't release downgrade or other exploits.