Yeah, but if he elevated a sitting Justice, he would have three confirmation hearings to go through instead of two. Democrats in the Senate probably wouldn't be able to stop the elevation if they opposed it (however imprudently, in my opinion), but they could certainly slow things down, force the administration to expend more resources than it would like, etc. It would be, and will be, marginally easier for them to go through only two confirmation hearings. It will still take quite a bit of resources and political capital to pull it off as it is. If I were Justice O'Connor I would keep doing my homework; I just don't foresee her being able to effectively retire anytime soon.