My Buffalo bought end of 2010 was pre-installed with a version of dd-wrt (a.k.a the “Pro” firmware of Buffalo which was installed by default as of the second half of 2010 instead of their ‘friendly’ firmware).

1. In the dd-wrt router database, I have searched for WZR-HP-G300H and found it was supported by v24-preSP2.
2. I went to the file server where dd-wrt firmwares v24-preSP2 are available.
3. On that file server, I went to the more recent sub-folder (/2013/02-11-2013-r20675) and looked for the folder “buffalo_wzr-hp-g300nh” and fetched the firmware “wzr-hp-g300nh-dd-wrt-webupgrade-MULTI.bin” (The other one “buffalo_to_ddwrt_webflash-MULTI.bin” is to upgrade from the Buffalo ‘Friendly’ firmware – which used to be installed by default before the second half of 2010).
4. Using my laptop connected via wifi, I went to the “firmware update” page of my Buffalo, via its web interface menu “Administration > Firmware Upgrade”.
5. I chose “don’t reset”  after flashing, selected the firmware downloaded previously on my laptop and clicked “Upgrade”. It took quite a few minutes.
6. Next, the countdown being completed, I have refreshed my browser and the new interface appeared. The previous one was red with “Buffalo” displayed on top. The new one is mainly black/blue and yellow without Buffalo displayed anymore.

Everything was still working fine immediately after the upgrade, without any lost of settings. I.e. mainly (from my point of view):

• The Buffalo IP address, which was a custom one, didn’t change.
• The Port Forwarding where still configured in NAT/QoS > Port Forwarding.
• The wol addresses where still available in Administration > Wol.
• The Static Leases of the DHCP Server where still configured in Services > Services.

dd-wrt wiki for WZR-HP-G300NH : http://www.dd-wrt.com/wiki/index.php/Buffalo_WZR-HP-G300NH

Buffalo : http://www.buffalotech.com/products/wireless/single-band-wireless-routers/airstation-highpower-n300-gigabit-dd-wrt-wireless-router