What is the operating system? And what motherboard too actually, because I'd like to know if maybe you are seeing the AHCI screen and just thinking it's asking to switch to AHCI, or does it actually ask?
If you are using windows 7 or Vista you can easily switch to AHCI if you installed in IDE Mode, you must be using one of those operating systems, and you must be using the Intel ports. Then all you have to do is make the following registry changes, then reboot to the BIOS and set AHCI and then save/apply
Enable switching between all IDE/AHCI/RAID modes by changing "Start" Values in these keys to 0 (Win 7 / Vista ONLY)
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci\Start
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Pciide\Start
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\iaStorV\Start