So I replied to another thread you made at the end of January...and this will be gentle necro-threading, too. But I'm posting some of the things I've learned having the same issues as you for posterity so that the next person who's trying to google their way to victory can have a little more help.
"ChangeScreenRes=" only alters what's available to you in the Video Modes section of the Setup menu in the game. It doesn't help with preserving your desired aspect ratio or resolution or anything. With the variable set to "0", you'll see the resolution you've set in conf.ini (in my case, 2560x1440), but then the only other resolutions available in the drop-down menu will be 640x480, 800x600, and 1024x768. Also, the Full Screen/Windowed toggle will be automatically set to "Windowed" regardless of what you do in conf.ini. I replicated what you did and tried to set the toggle to "Full Screen" while the conf.ini setting was "ChangeScreenRes=0". I applied the change and went back to the main menu and the game instantly crashed.
ChangeScreenRes=1 allows the Full Screen/Windowed toggle to work properly, gives you the full range of resolution options in the game drop-down menu, and also gives you those resolutions at different color depths. So, aside from two different color depths on a ton of resolutions that I don't need to use, the game allows the choice between 2560x1440x16 and 2560x1440x32. With ChangeScreenRes=0 set, the resolutions will be limited to the ones I've noted above, and will also have no color depth appended. AKA 2560x1440 is shown in the menu with the variable set to "0", and 2560x1440x32 is shown with the variable set to "1".
BTW, the Color Depth selection is the same as the "ColourBits=" line in conf.ini. 16 or 32 are your options.