If you install WinXP from scratch, then you press F6 near the beginning,
and offer a floppy with the AHCI driver on it. That's one way to do it.
That is what Intel wants you to do.
What you have probably been researching, is the "chicken versus egg" solution,
where you look for a way to edit something, such that the driver is changed
on the fly. The driver is set up in such a way, that it cannot be installed
on the fly. If you switch BIOS mode first, the computer will not boot (you would
get an "Inaccessible boot volume"). If you try to install the AHCI driver
without the BIOS mode changed, the driver will not install (because the PNP
information does not match). There are "recipes for editing stuff", which
get around the problem, and jam in the driver. It sounds like that is the
route you are looking at.
There is a third way. It would involve adding another storage
controller to the machine, such that the C: drive is moved to
another controller. At that point, the system is no longer
dependent on the Southbridge driver for booting. Then, you can
change the BIOS setting to AHCI, reboot (now, rebooting from some other
hard drive controller), install the AHCI driver, reboot, then
shut down and move the disk back to the now-AHCI-equipped port.
I call that a "bounce install" for lack of some other term for it.
Because you bounce the hard drive around from port to port, until
the problem is solved.
I think I have done that in the past, with a separate Promise card.
As part of moving an OS from one machine to another. The card traveled
with the disk, so some bootable hardware that was the same on both
machines, would be present so it could be booted.
Perhaps in this case, a Silicon Image SIL3112 card could be used,
until the Intel driver is changed.
The BIOS setting just changes the Plug and Play number returned,
when the new hardware wizard checks. The BIOS is hardly affected
by all of this. The only time the BIOS can be an issue, is on
some pre-built machines that do not have all the settings they
should. My laptop for example, has a pretty empty BIOS, and I
think in fact, the only setting inside it, may actually be
to switch from AHCI to vanilla IDE flavor of SATA. And that
is for cases where you might have installed an OS that did not
have an AHCI solution.
In the case of Windows 7, it is not like WinXP in this regard.
On Windows 7, you re-arm the driver detection with some registry
settings, shut down, change BIOS settings, and Windows 7 figures
it all out on the next reboot. So it is not nearly as traumatic,
except for the need to work in regedit. They did not make an
easy button in the OS like "driver re-arm" or anything. In
Windows 7, a number of default drivers are present, which is
what is used at driver sniffing time. MSAHCI, IASTORV, and the
like. People mess around with that stuff, while trying to get
TRIM support for a new SSD replacement.
I bet there is at least one backup solution, that supports
moving a backup to a different hardware setup, and that
may be yet another solution. Some backup software, promises
to allow restoring to different hardware, which might be
another way around the chicken versus egg part.