Actually, Andrew, it is impossible to change the resolution of the game while in game. The option is greyed out, cannot be selected. The only solution I have found in the Hi-Res world is to reduce my screen resolution in Windows before I launch CK2. With that done, the next problem is that the in-game cursor is enormous, covering parts of the mouseover text.
The game might be six years old - but plenty of DLC has been added over the years, and more is yet in the pipeline. It seems to me that it would be well worth it, from a market standpoint, to enable CK2 to fit into the high res screen world. My guess is that probably 50% of the user base, and a substantial number of people who've never played the game (because they're just coming into the market) will be much more interested in purchasing the game and DLC if it scaled in the increasingly predominant hi-res market segment.