I agree with the above. I’d tell them sooner rather than later. If they like you they will still want to be your friend. It would be strange to not want to hang out with someone because they won’t be around the following year. But what WILL make them upset is to feel like you kept a big secret from them long after you made up your mind OR if you let your roommate assume you’ll be living together next year.
I would probably tell them you got this offer and were waiting to see if you met the requirements to transfer before making up your mind. I assume you had to get certain grades or something?