Why isn't the original design for the Yorktown being used.The original design that was going to be used had the warp engines below the saucer.
I do not remember who originally designed it, but I thought it was more unique and interesting to look at.
Two versions of the USS Yorktown appear in this film. At the beginning of the film, it appears as a Constitution Class starship (same design as the 60s USS Enterprise). During the film, it is given a "refit" after it is damaged in an attack. The refitted ship (designed by Andrew Probert) has the warp engines below the saucer. If you watch the teaser trailer on Youtube and Vimeo, you will see both versions of the ship appear in the trailer: https://vimeo.com/43588987