At present, DIR-868L can only be converted to RT-AC66U_B1 or to RT-AC68U.

Besides the problems mentioned in the opening post such as LAN/WAN ports rearranged and LED not working normally,
1) If converted to RT-AC66U_B1, everything works fine except 2.4G WiFi very slow
2) If converted to RT-AC68U, everything works fine except 2.4G WiFi very slow and WiFi is disabled/off after every restart.
Here's the guide to fix both 2.4G WiFi speed and WiFi disabled after restart. It will be converted to AC68U. AC68U has slightly more info when viewing connected clients list compared to AC66U_B1.

Only do this after you're already fully converted and running Asus firmware.

1) Download attached file. newcfe68u.bin and newcfe66ub1.bin and mtd-write and CFEEdit

2) Use CFEEdit and change 3 macaddresses and WPS Pin according to your own router in the CFE file.

3) Turn on the SSH from router page. Administration > System > Enable SSH > Apply

4) Open WINSCP. Choose SCP protocol, enter router IP address, username and password

5) Now decide if you want your router to be Asus AC68U or AC66U_B1. Drag and drop both newcfe68u.bin OR newcfe66ub1.bin together with mtd-write file to the right side of WINSCP

6) Use Putty and connect to router. Enter username and password

7) Enter these commands in Putty to flash new CFE



8) Enter this command into Putty to clear NVRAM and reboot



And it should startup as AC68U or AC66U_B1.