I don't think graphic options (other than windowed mode and resolution) will be locked in alpha, although they will possibly be limited.
I do think certain things will be very adjustable though. They wouldn't want to mix people with bad specs and good specs otherwise. If there were close to no options to change, the game would either run extremely slow on low-end computers, or wouldn't be possible to make it look as good as people want on high-end computers, and I'm sure this is one of the things they want to test in alpha (as well as beta). Although they may just push the graphics down to the minimum requirements throughout the alpha just to make people test the game rather than graphics.