How do I tell my current roommate (and friend) that I want to live with someone else when the lease is up?

I am currently living with a friend and one other roommate. The friend has assumed that I will live with her next year. I have discussed living with another friend, and don’t know how to tell my current roommate. While this seems simple, it is slightly more complicated—I have a dog, whom is not allowed to live with me right now because my current roommate/friend is allergic (even though he is a non-shedding, hypoallergenic breed which was chosen specifically because she has allergies). I have told her that whatever my living situation next year (my last year of college) he will be with me. She is now telling me that this is fine, she just doesn’t want him to have “accidents” and she will not be responsible for taking care of him in any way if I am gone during the day at class or work (which includes letting him out to the bathroom). I understand that it is not her responsibility to do such things because he is my dog, but it is also not fair for him (and me) to be punished if he is not let out to relieve himself and thus has an accident in the house. I am hesitant of the situation because I was told before signing our current lease that I could bring him, and then told NO 5 minutes after signing the lease. My other friend is an animal lover and would make for a much better living situation. I just don’t know how to bring the subject up with the current roommate/friend (whom I would like to remain friends with, but just not live with). I am obviously not a confrontational person, so how do I do this while being firm in my decision, but not harsh? Advice?