Target change. From ar71xx to ath79.
There is no clear update path.
You may need use console sysupgrade command and override the image check (as the 18.06 LuCI offers no "force" option like 19.07 and master).
Hi here's what I did when flashing my c7-v2 from 18.04 to 19.07-rc1. installed winSCP and putty on my pc. Loged on to my c7 using winscp. Renamed the bin file for 19.07 to openwrt.bin. Uploaded the openwrt.bin file to /tmp. Then loged on to my router at 192.168.1.1 using putty. Then type. "cd /tmp" then "sysupgrade -f openwrt.bin"
Alternatively, you could probably flash from 18.06 to 19.07 ar71xx, and then flash to 19.07 ath79 with the option to force the installation through GUI.