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.
Get the OpenWRT image onto the router. e.g.
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
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!).
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.