Glad to see your post because I was in the exact same position a few months ago. I was attending a selective/'top' school [school X] until this past May, and I was completely miserable. Like you, academic performance was not a problem--there was nothing that pushed me out. I just wanted a completely different atmosphere and social experience.
Right now I am still waiting on my transfer decision from my flagship state U, but I am definitely not going back to school X, nor to schools of its kind. It took me a while to stop being defensive--to myself, I mean--about the decision. Pride was a huge barrier to 'down-grading.' Not really because of cache or prestige, because I turned down a couple even better schools to go to school X, but because I still wanted to be in an 'intellectual' environment. A summer away has made me realize that you can find or create a stimulating environment anywhere. I also realized how snobbish I had become, going to school X for two years....forgetting that not all smart people get to go to top schools. Money, location, major plans and the crapshoot game of admissions can all influence a student to go to a flagship U. Plus, due to their typically large size, flagship state U's can offer every single type of person and personality.
Of course, I feel a bit sad when I realize that a chapter of my life is ending. Mostly I feel this way when I look at the course catalog for my flagship state U. The more 'vocational' nature of the classes offered is going to be totally different. But at the same time, just like the diversity of the students, the size means more diversity in classes (Real classes about people of color! Expanded language options! Multiple sections!). And I look forward to earning a degree in a more 'vocational'/applicable environment, where the default graduate situation is not unpaid internship on parents' dime. Sometimes those top colleges are the true dead ends, unless you have a lot of cash being thrown at you.
It sounds like you've figured out what you want that is currently lacking: "a more vibrant social scene and more school spirit." It also sounds like you know that your state U can offer those things. To me (as a completely biased party), it feels like a good plan! But I hope you take some time to think about it. I thought about it for over a year, and that has definitely helped me avoid most "what if" moments. If anything, I like to think of it this way: I'm getting the best of both worlds--two completely different college experiences in one degree! Plus, at risk of sounding corny, I'm following my heart (and NOT the college confidential-style race towards prestige!).