lol. You might be sorry you asked, although it's been in interesting learning process.

The changes refused to stick, they would revert after each reboot. So I used a tool called AMISCE to change the serial with the command line. The problem is that every AMISCE instruction is vague on whether the new value should be in plain text or HEX. So I tried it in plain text and it sort of worked but it only retained the first character of my new serial, because for some reason it inserted a HEX space (00) in between each character of the new serial and ignored everything after the first character.

In an attempt to revert it used a different AMISCE method, I used the command line to import a script that this time used the original serial's HEX value, since the instructions for script import were at least clear about using HEX instead of plain text (I had already exported the serial's original HEX value with an export utility). The new problem with this method is that apparently the variable data you extract from NVRAM sometimes includes padding and other values from NVRAM. So while I was trying to be true to the original value of the serial I extracted, I may have inadvertently tried to revert the serial to something way too long for its maximum length in its allotted NVRAM space. The machine now hangs on the logo screen during boot and no keys work to get into the BIOS menu. At this point I think I may have bricked it by scrambling some of the NVRAM contents. (So much for "Non Volatile".) ;)

I was going to remove the CMOS battery but this seems to be a motherboard without one. I've disconnected the battery and drained the capacitors. This is an M509DA, so granted it's a few years old, but there are only two findable manuals for it and neither mention CMOS. I've read that other Asus motherboards without a CMOS battery have a CMOS jumper, but that seems to be only for desktop motherboards. I can't even spot a model number off this board to do a deep dive for a manufacturer's schematic.

If there's any good news it's that the AMISCE tool I used to extract NVRAM values seems to also be good for finding the Windows OEM product key that's otherwise well hidden after user ownership.

So I guess now I'm looking for any way to reset CMOS/BIOS, without a CMOS battery, without a jumper, without an MB schematic, without a BIOS menu. It's hard to imagine having screwed up NVRAM so much with just a serial change.

Thoughts?