I am only a freshman, and barely that. But I have read and started a lot of these sorts of threads, and it dissatisfyingly boils down to this: you have to decide for yourself.
I think all kinds of engineers are involved in building these things, so it really depends on what part of it you want to do. If you are just absolutely dying to do whatever specific things that the real aero guys do best, to the point where you just feel like you are settling for a life you don't want, my opinion is to throw the dice a little and do what you like. But it's also my opinion that not a lot of us experience that kind of I.D. foreclosure at 18. Beyond that, unless you specifically know the exact thing you want to be doing with planes, and want to ask experts if a MechE is often qualified to do that, I don't think anyone can give you much here.
Good luck with your decision, and you should have at least another year and a half to make it.
(Personally, I am almost considering not doing engineering at all. I don't know enough about it yet to say, and I will probably still do my undergrad in MSE, but I am starting to come to grips with the fact that I never wanted to be an "engineer" the way the job is usually described, I want to be a scientist. And as I am starting to figure out that I have some ability to succeed in school and could go to grad school, I am also starting to feel like I really want to be in a lab designing kooky experiments. And letting myself think about doing a PhD in PChem or something less employable makes me happier. I still don't know what I'm going to do, because I'm not sure what I like or what does what. But I know I don't want to grind out my life, or even the next 4-5 years, doing something I don't like, just because it seems like a safer career. If you think you don't know what you want to do, or aren't sure, or learn it really is a smart way to get where ever you decide you want to be, then that's great. But don't do a degree you don't want to do unless you really don't care either way.)
Edit: I think you could also do Mech undergrad, take as many Aero electives as you can, and if you like it, do an aero MS orPhD or something.