I'm going to change my plans with someone who wanted to room with me.
What made you want to room with them?
There was a girl I wanted to room with but I don't want to do this anymore.
The other person was counting on you and changing your mind could lead them to be in a bit of trouble. Work with them to make sure they have alternate plans and come to a mutual understanding.
I should make them miserable for not being my friend anymore.
You still need to work things out with them. They were counting on this and you need to make sure everyone gets to find a place to live and if not you should stick to your plan.