The rights to Age of Chivalry we own, the rights to a source engine license we do not and never have. Anyone can create a mod, to go commercial you need the license. That is part of the reason that we switched to UDK for the new project, because source engine license was denied to us. Although we're now much happier for it .
If your computer is from 2004-2005, you can run Unreal Engine 3 games. Albeit, with tweaks and low settings, but playable.
Also, I dunno if I'm happier for it. I love Source, despite it's flaws. Sure, the newer videos look insane, and will surely be better than AoC ever was, especially the hit detection, but still...I've never found a Source based game I did not enjoy.