If you’ve never seen Rodgers & Hammerstein’s much-beloved “Oklahoma!” — I hadn’t — then Arena Stage’s swell new revival of the 67-year-old musical may surprise you. Not because director (and Arena Stage honcho) Molly Smith has cast people of color in several key roles, including the two main romantic leads. (“The country’s changing, and we got to change with it,” remarks Curly, the kindhearted turn-of-the-20th century farmhand played by Nicholas Rodriguez, near the end of the show.) And not because the songs, performed by a 14-piece orchestra and a cast without a weak set of pipes in the bunch, reverberate thrillingly through the Fitzchandler Theater, the acoustics of which have been given as thorough an upgrade as its appearance.