I never upgraded my arena but my lower-tier seats became bleachers and my courtside seats became lower-tier. Only now I realize that. I thought that was going to happen only for "seats you have built (or are in the process of building)", like it says in the News page.
I used to have 4500:450:50:0 and now I have 4950:50:0:0. Lost some money because of this in the last 2 home games.
I was already unhappy witht the arena changes, since I had no idea it was going to happen, otherwise I would have upgraded before my arena... And now I'm even more concerned.