Enable /system write in TWRP - Systemless root

Moto X Play (XT1562), SuperSU 2.78 (systemless root), [TWRP 3.0.2-r3](http://forum.xda-developers.com/moto-x-play/orig-development/recovery-twrp-2-8-7-0-t3194656) * 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](http://forum.xda-developers.com/moto-x-play/orig-development/recovery-twrp-2-8-7-0-t3194656/page37#post69124377)