Ick.
They just couldn't resist dinking with the design.
Recently they've been "reworking" old Star Trek episodes, cleaning them up, and replacing some of the old effects with modern ones. For the most part (at least in my opinion), they have done this with great respect for the original work. The "Enterprise" in those episodes looks like a cleaner, more realistic version of the original model. Nothing more, nothing less.
I don't know the plot of the movie, but I have no idea how they're going to work the "Enterprise" into the original continuity of Kirk's career. He spent a lot of his post-academy time on other ships.