A simple registry edit will enable a hidden option in the button settings which will allow you to change the function of camera button.
Using a registry editor, navigate to the following key. (If you are unsure as how to do this, use this forum's search function to look for guides on how to edit the registry. I use Resco File Explorer, which isn't free, but there are plenty of free registry editors.)
You will see an entry called NoHardwareButtons.
Change the value from 1 to 0.
Now if you go to your button settings (Start - Settings - Personal - Buttons) you will notice a new option which allows you to assign the camera button to open any app you want. You can use this option to restore the button back to the camera if you want.
EDIT: I've created a cab that should change the setting for you. This is my first cab, so please let me know if it works or not.