There is no driver change, only changes in the way the device is supported by the kernel (mach files in ar71xx, Device Tree in ath79)
The "ar71xx-ath79" target is used in the wiki for devices that are supported both in ar71xx and ath79, there should be 19.07.0 images for both.
In any case, use ath79 for new devices, and migrate from ar71xx to ath79 on existing devices.
There is a guide here: https://openwrt.org/docs/guide-user/installation/ar71xx.to.a...