Buffalo encrypts their firmware (sorta) so that only the User Friendly and Buffalo Branded firmware (currently installed) will take them. I however found instructions on how to strip the encrypted header from the firmware.
I've used this going from OpenWRT to the 14998 dd-wrt firmware.
You'll need an application called DD (comes standard in linux but is a needed download for windows users). http://www.chrysocome.net/dd?
dd bs=28 skip=1 if=c:/tmp/wzrhpg300nh-pro-v24sp2-14998? of=c:/tmp/firmware
?This command assumes you have placed the 14998 firmware file in a folder on your C drive called "tmp". It will strip the header and create a new file called firmware. Extract dd.exe to the root of C. Open a command prompt. Go to the root of C. Paste in the command. You'll then navigate to the upgrade firmware page of OpenWRT and use the new "firmware" file to revert to DD-WRT. I've only done this once so my instructions might not be 100% correct, but if I remember correctly, it's really easy compared to trying the TFTP method which I never got to work.
This has been the only successful and easiest way I've had to go from OpenWRT to DD-WRT.