Needing Opinions

I was just hired as a CNA on the Med/Surg floor. When I was interviewed, I was asked whether I preferred the day or night shift. I said nights, but would definately take the day position because that was what was avaliable at the time. They said that if a nights position opened up, I could transfer over. A couple of days later, they opened up more positions - one was nights on the postpartum floor. I really wanted it because of scheduling and because I want to be a midwife ) so I called her and asked if it would be possible to switch over to that department. My question is- should I have done that? I haven't started yet, but I would have a different director so I don't know if that would be more complicated. I didn't want to step on any toes, but I figured if I didn't say anything, that another position there might not come up for a while. What do you guys think?