I suspect the original drive will also work fine on the SATA-II port. The issue was simply that XP doesn't recognize SATA-III.
As for switching to AHCI "after the fact" ==> install the AHCI drivers; then turn on AHCI in the BIOS. Note that in a non-server environment there is VERY little performance difference between IDE mode and AHCI mode. So if you're expecting the performance to improve when you switch to AHCI you'll be disappointed.