IT should work setting rendering modes. Try both using the ogre.cfg and open it with notepad, then set the rendering mode there.
It should always be able to switch one way or another especially if you go in the game, change rendering modes, then restart the game, if it doesn't change the rendering mode in that little box that pops up with settings after an ingame change, close it down and run the icon once again. Really try different things.
Cause if you aren't able to switch between two rendering modes, something is either wrong with the copy of the game you have installed or the computer itself not able to use the given rendering mode.