Which ever hand you write with, goes at the top of the stick! This is the artistic side of the stick. The hand down the shaft, is the power side / leverage side of the stick.
If you make the change, your shot will only improve!
Heh. 690 career goals. 1033 assists. 6 Art Ross trophies. Now just think how much better Mario Lemieux's career would have been if only someone had taught him to shoot the right way! (Lemieux writes and golfs right-handed, and of course was a right-handed shot when he played hockey).
On a more serious note, I don't know nearly as much about this stuff as HeadCoach does, but I will make the claim that it's not as cut-and-dried as he makes it out to be. Regarding ambidexterity, I think people get caught up in a false dichotomy. That is, they think "either you are ambidextrous or you are not". I don't think that's true. I think there's a continuum. Some people have a strong preference for one hand or the other, some people have almost no preference, and many people are somewhere in between.
Personally, I'm somewhere in between. I do most things right-handed (including writing), but some specific things feel more natural to me left-handed. When I started playing hockey, I bought both a lefty and a righty stick. I alternated them from one practice to the next, to see which one I felt better with. I was interested and surprised to find that with regard to stick-handling, I really could not choose. Both lefty and righty felt pretty natural to me, and I didn't seem to perform any better one way or the other. Shooting was a different story however. I was okay shooting lefty, and I'm sure if I had stuck with it I could have gone on to do fine with it. But it definitely did not feel as natural as shooting righty.
So, a right-handed shot I became. As a partially (mildly) ambidextrous person, I feel very comfortable that in the end I made the right decision for myself. I think the biggest caveat for a person who plays hockey with their writing hand on the bottom of the stick (instead of the "correct" way, with the writing hand on top) is that you need to make sure you are using good stick-handling technique. You can't let your bottom hand do more than it's supposed to. For me it doesn't seem to be much of a problem though.
edit: I should mention that when I originally bought those sticks, there was a hockey coach who happened to be in the pro-shop, who was helping me decide. The test he did was, he said "If I were to hand you a hockey stick, which hand would you be inclined to grab it with?" For me, it was my left hand (which surprised him, since I am mostly right-handed). So he ended up being the one that suggested I buy both lefty and righty sticks, to try them both out.