Actually, every movie studio hurts for money, because for every Dark Knight, there's 20 complete flops of varying cost, including a supposed blockbuster or two. No movie studio can sit back and say "we're not hurting", because it never knows when the next Waterworld is going to bite it in the arse. Also, the last Potter movie cost somewhere between 150 and 200 million, and it only made average Potter movie money (in other words, it didn't make much more than the previous Potter movies did), so the costs are up and profits down (in the most technical manner - worldwide grosses still mean the movie tripled its budget).
All that said, I agree that you don't move a movie with such a well-advertised release date just to try and make more profit. I looked at the release dates of the other films and there's no pattern of the summer releases making significantly more than the fall releases. POA actually made less than the others, and it was released in the summer. The Industry was ready for Potter to come out in the fall - I just got my Entertainment Weekly Fall Movie Preview issue this week, and there's Radcliffe's giant face on it. So....
I'm thinking there has to be other reasons for the move, then. Either a problem with the movie itself or the post-production; maybe the DH movie set isn't going well, and they decided to make less of a gap between HBP and DH. Maybe there's some evil political shuffle going on behind the scenes and this is some power move. I know I sound like a conspiracy theorist, but I agree that there has to be more than "profit" behind this move.