Wait for other responses or perhaps someone more experienced may confirm what I am suggesting. I think doing all of these changes in safe mode is going to work best.
Type "msconfig" in run. Select the Boot Tab and choose safe boot. Reboot and enter bios during that reboot. Change to AHCI Mode and any other changes needed. Save and let the system boot into safe mode. Perform the changes needed. At this point you can probably reverse the "safe boot" in msconfig but if you want to be safe you can continue in safe mode. Reboot and enter bios. Set bios back to RAID and any other changes. Save and allow to reboot. If you opted to stay in safe mode to this point go ahead and disable it in msconfig and then reboot.
There are other ways to get into safemode obviously but this way keeps it active until you are ready without having to choose safe mode each boot.
Hope this helps. NOTE !!! I am not sure about editing the partition. I believe it is possible to edit a partition without losing data. What I outlined above is mainly to prevent an issue during the switch between ACHI and Raid.