Upgrade from openwrt
From DD-WRT Wiki
I had a linksys WRT54GS version 2 router that I wanted to upgrade from openwrt to dd-wrt. I could not find any documentation on how to do this. So I created this page just in case anyone else needed to do this.
Replace your firware image referenced below with the correct image for you router.
secure shell or telnet into the router running openwrt
mtd -r write dd-wrt.v24_mini_generic.bin linux
If all goes well your router will reboot, and when it comes up it will be running dd-wrt
I did a factory defaults after the reboot, I am not sure if it is needed or not.