The Hulkbuster suit makes sense, but the suit switch in the final third of Age of Ultron was probably the most gratuitously dispensable one of the entire IM/Avengers series. At least in previous films they gave a reason, no matter how flimsy, as to why Tony switches to a new suit. AoU didn't even try. A throwaway line would've sufficed.
I'm willing to bet that most general audiences didn't notice the difference. If I weren't such a nerd about these things, I doubt I would've either, they're fairly similar at a glance.
^ but looking at them closely, I wish they had just had Tony use the newer suit on the right for the entire movie. It's slick as hell.