I was just about to ask that -- it's why I posted the heads-up in the thread for the demoplay fix a few days ago. The way that PC 1.0 hooks into the client depends on it being exactly the version that is expected, so hex editing parts of it change the signature of the file and cause it not to be usable.
The best solution is to call your hex edited version something else, like CohDemoplay.exe or something.