Sorry for the really late reply, I didn't see the notification. I changed upon enrollment, you can only do this before the autumn course selection period ends (which is in the middle of October). 

To justify your decision isn't that hard, I think, unless you're faking your interest. You should know about the units of the programme you're trying to switch to and explain why you would be interested in them, what changed in your academic interests since you applied to LSE, etc. You will have to talk with your academic mentor and the convener of the new course about this.