With version 2.5.5, Hipchat replaced its icon. I did not like it, so I decided to change it back to the previous one. Along the way I found a cool project which updates the Hipchat's visual style by Wells Riley. You can find it here: http://wells.ee/hipchat. I decided to keep the blue icon, though.
This will work ONLY on OS X.
git clone git@github.com:marcinbunsch/hipchat-restyle.git
cd hipchat-restyle
makeThis will close Hipchat if it is running, replace files in Resources, then start Hipchat again.
The original Hipchat icon is taken from the previous Hipchat version.
Updated Hipchat images are by Wells Riley (http://wells.ee/hipchat)