Enable /system write in TWRP - Systemless root

Moto X Play (XT1562), SuperSU 2.78 (systemless root), TWRP 3.0.2-r3

• When I flashed TWRP, I chose the option of not writing to system to enable OS update (and also made a Nandroid just after flashing it as a backup)




• Now, I made some minor changes to system audio files, without dwelling on consequences - won't be able to update OS ( update expected this quarter) without flashing stock OS

I want to enable /system write option in TWRP ( without flashing TWRP again ) so that :

a) Flash backup Nandroid ( of stock OS from the first Nandroid), if I decide to upgrade OS

b) If I don't want to upgrade, have a complete Nandroid and tweak further knowing that I can safely roll back changes

In other words be able to backup and restore /system along with data

Is it possible and how ( please provide detailed instructions )?

Edit: Flashing different versions didn't help - see my unanswered post on XDA