but it will crash in boot time.
you can go ahci to ide but not from ide to ahci (especially when you have only one io controller onboard that is southbridge to which your hdds re connected), it will not load (atleast not normally) and may corrupt your os
since you are using window xp which needs to install extra drivers during windows installation to use ahci or raid which i think you didn't thus you will need to reinstall your os with ahci or raid driver
without all these drivers windows xp does not detect ahci or raid devices at all
though if you are not planing to do raid then it doesn't worth extra hassel