It is suggested to enable ahci in Windows 7 Before you switch the bios setting, here is how.
Changing from IDE to AHCI
In regedit navigate to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Msahci,
click "Start" in the right pane and modify value to 0
Do the same for
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\IastorV
Changing from RAID to AHCI, you need one more step.
Msahci driver requires Atapi.sys miniport to be loaded during boot as well. So you need to change theStart value to 0 for both of the following entries in your registry. You can use regedit.exe utility.
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\msahci
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\atapi
Restart the PC, enter the bios and enable AHCI, save changes on exit, boot into Windows, it will install necessary drivers.
Restart PC again
Done.
Now you can install the new SSD and W10, then if you want to boot off the old W7 drive it will work also.
Source You may have to log in or create account to see the Microsoft source page.