AT attachment (or ATA) is a standard that has been designed for quite some time, which is meant to create an interface between the Central Processing Unit and storage devices such as hard drives. This ATA has been improved into two kinds, which are Parallel ATA (or PATA) and Serial ATA (or SATA). AHCI, on the other hand, stands for Advanced Host Controller Interface, which was designed especially for Serial ATA.
It helps in the passage of information between the host and the storage device. AHCI is a new host controller interface, and it does not work with the old ATA. It is only compatible with hard drives and motherboards that are SATA enabled.
It will most likely be impossible for you to switch to AHCI after you have selected ATA for your windows installation if your PC is not having the compatible installed drivers, this leading to the damage of your system. To easily switch from ATA to AHCI, you will have to install your operating system afresh.