I had this very issue some time ago. In my case, this Reddit post helped.
Basically, to migrate to the traditional system root, you need to have the TWRP recovery installed, the latest SuperSU flashable ZIP, your phone to be already systemlessly rooted and SuperSU by Chainfire to be your root manager app.
Next, boot to Android, open SuperSU and navigate to the Settings tab. Find the Full Unroot button and tap it. When SuperSU asks if you're sure to unroot, accept. When it asks whether to restore the old boot.img, deny.
When the phone reboots, input the appropriate keys and boot into TWRP. Once there, make sure that the /data partition be mounted. Then, under the Advanced menu, start TWRP's Terminal and issue the following commands:
echo SYSTEMLESS=false > .supersu
. Alternatively, you can use
echo SYSTEMLESS=false > /data/.supersu
to achieve the same result. This creates the file .supersu inside /data.
Finally, flash the SuperSU ZIP to force SuperSU to use the standard, old root procedure.
The .supersu file
This file is checked (e.g. sourced) by the SuperSU ZIP. If it finds the entry
SYSTEMLESS and it has value
false, the installer will perform a system install. If the value is
true, the installation will be systemless.