If you are using a physically based shading pipeline and physically based values (such as light intensities), adding HDR support is not a big technical task. However, changing the tone mapping operator changes the look of the image. You need to tune camera exposures and eye adaptation, tweak color corrections, tune your bloom, god rays, tweak bright particle effects (such as lightning), etc. If you have lots of content, there might be lots of tweaking to be done in order to make the image match the artistic goals. Content tweaks are going to take the most effort in porting for HDR. But developer who spend lots of time tweaking their content for HDR are also going have a stunning looking game at HDR. Let's see how big HDR adaptation will be. Stereo 3d was a big buzzword a few years ago and only a few developers supported it. HDR is much better, but with so small amount of HDR televion sets around, not every single developer is going to spend extra money to support it.
