If anyone has done this on an Optiplex 780 where the OS was installed under Legacy and they successfully switched to AHCI, please let me know what you used to accomplish this. I tried downloading the Intel PreOS install drivers for the it but as soon as I try updating the 2 drivers in question, it gets about halfway thru transferring the files and BSOD. I am running WINXP SP3. I also tried uninstalling those components and rebooting hoping it would ask for new drivers and that didnt work either.
Not possible under win xp as far as i know. i`ve always gotten a crash. Perhaps it is possible if you do a repair installation with a windows disk, after backing up just in case. It might be just a matter of changing some setting, but in my experience letting windows repair detect the old installation and install changes will usually help to keep files and programs while changing disk access mode.