You should build a firmware with unlock the art partition and change it.

If you use OpenWRT-18.06 or glinet repo, you can edit the file target/linux/ar71xx/image/generic.mk, and remove ro flag for art. It would looks like:

define Device/gl-ar300m
  DEVICE_TITLE := GL.iNet GL-AR300M
  DEVICE_PACKAGES := kmod-usb-core kmod-usb2
  BOARDNAME := GL-AR300M
  IMAGE_SIZE := 16000k
  MTDPARTS := spi0.0:256k(u-boot)ro,64k(u-boot-env),16000k(firmware),64k(art)
endef
TARGET_DEVICES += gl-ar300m

And compile it. Finally, you should write a shell script to change this position.

image