USE THESE INSTRUCTIONS AT YOUR OWN RISK
I have only done this procedure once and sharing because of the lack of information on the web on how to do this.
Some of the sellers of the TL-WR703N are shipping the routers with DD-WRT pre-installed – some with a Chinese interface, some with an English interface.
Regardless, you need telnet or SSH access to the router as upgrading via the web interface will not work.
Step 1 :
Get the OpenWRT image onto the router.
root@DD-WRT:/tmp# wget http://192.168.1.142/openwrt/ar71xx/openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-factory.bin
Connecting to 192.168.1.142 (192.168.1.142:80)
openwrt-ar71xx-gener 100% |*******************************| 3840k 0:00:00 ETA
Other methods can be found here :- http://wiki.openwrt.org/doc/howto/generic.sysupgrade
Step 2 :
The TRICK is that the partition names are different between OpenWRT and DD-WRT. Whereas all OpenWRT instructions will tell you to write to the ‘firmware‘ partition, this does not exist on DD-WRT and you have to use the ‘linux‘ partition instead. Use the ‘mtd’ command as per the example below to write the OpenWRT image onto the router. Note the ‘-r’ argument will reboot the router as soon as the flash is complete. (As usual, do not power off or disconnect during the flashing!).
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root@DD-WRT:/tmp# mtd -r write openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-factory.bin linux
Unlocking linux ...
Writing from openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-factory.bin to linux ... [e]
Connection closed by foreign host.