NuKeR said:
lol RegularX, the UT2003 engine is old compared to the current Unreal Engine build
oh lol - that's not the point.
We were discussing whether the engine can be updated in 2k4 and still play 2k3 games.
CliffyB's comment was that the UT2003 engine was now faster and more efficient for UT2004.
Faster and more efficient almost undoubtably means native code updates. IOW, the Engine code itself. No, we're probably not seeing new lighting tricks or new texture tricks, but instead more efficiency in the same renderer which lends to lowering the bar on the hardware requirements. Which probably means that your current hardware will be able to push better settings.
If that's true, which by Cliffy's statements it sounds like it is, and the UT2003 compatibility remains, then the answer to "Can you upgrade the engine and maintain backward compatibility" is "yes".