Hi Kelly,
Is your concern "should you do it?" First off, how old are the children? Have you discussed this with your fiance? I can understand you wanting to change the room and I think it is a reasonable idea. I don't see why there should be a problem. I don't know how old the girls are, but you can explain to them that you need to use the room when they aren't there. Hey, it's your home.
Goodluck, Brooke