I guess since the two boards are from Asus maybe they are close enough to the same thing for them to use all the same BIOS firmware. It is possible.
I would be interested to know if the exact same file is listed as a usable BIOS update file for both boards.
There is a kinda interesting formula that Windows uses to determine when hardware changes can cause you to need to update Windows. You can look it up if you want.
You basically need 4 "changes", and technically every part is counted for a possible change, but the RAM, for instance, looks like this:
64 MB = 1
128 MB = 2
256 MB = 3
512 MB = 4
1 GB+ = 5
So if you upgrade from 64 MB to 128 MB it will register as a hardware change. If you went from 64 MB to 1 GB it would also count as one hardware change.
If you went from 64 MB to 128 MB, booted into windows, then upgraded to 256 MB and booted into windows, then upgraded to 512 MB and booted into windows, then up to 1 GB and booted into windows, that would still only count as 1 upgrade.
Your computer checks everything against the original install, not any point later.
If you, however, started with 2 x 2 GB (common today) and upgraded that to 2 x 4GB (also common) that would not count as an upgrade. Both register as the same point value. Basically it is almost impossible today for a Windows install to recognize a RAM change and count it toward product re-activation.
Anyway, it can record a lot of things and 4 things in total must change for a reactivation, but some things count and some things don't due to the way it is programmed. MSFT never brought some aspects of this into the new millennium.
It is possible that it registered you using the drive in SATA and changing it to IDE with the exact same physical hard drive as a hardware change, strangely enough.
None of that really helps you, but you may be amused to learn more about the process.
As far as helping you is concerned...
Is there any specific reason you cannot reinstall Windows, or is doing so just undesirable?
That being said, the following is a thread where someone named Roli wanted to change from AHCI to IDE and the reason given was that it would be faster with IDE.
http://www.vistax64.com/general-discussion/271218-switch-ahci-ide-tutorial-howto.html
I don't know, maybe my understanding of the subject may be off based on that grounds so you may want to verify yourself how you would like this to be.
If you want to bring it from IDE to AHCI, then maybe you can do the opposite of what they said to do in here and change everything from 1 to 0 that they are changing from 0 to 1 and so on.
That being said, I think you will find it to be a major pain in the a@@ to try to pursue this effort.
It would probably be easier to just reinstall windows.