You have not got an answer for this situation because it is rarely done! It would be my guess that the MS when you call may never had done this before either. I would think that since you have both reservation numbers it would not be a problem.. If you really think about it the person who has the resevation with the later date would just be asking for the same room as the first, and by doing that he would not be moving or controling another reservation except his own. It would be similar to just asking for room number X for example. I really went off the edge here with this-what I would do is just call down and do it. If MS says it can not be done than just ask for a supervisior, explain your situation, they can do it I am sure, with a lot less thought than I just put into this.