If you and your requested roommate are planning on living in a new room then it is your requested roommate's responsiblity to explain to their roommate that they may receive a new roommate or they can request a new roommate from the Housing Office.
If you want to move into your requested roommate's room and they currently have a roommate you will not be able to move into that room until the current roommate has requested a move and has completed said move with approval from the Housing Office.