From the much-clearer teaser, it looks as if it is being assembled in distinct piece -- saucer, nacelles and struts. While in the final shot, it appears as if the nacelles and saucer are locked into their final place; I doubt that they are. From other shots in the trailer, it seems that several pieces, like the struts that hold the saucer and the nacelles onto the secondary hull, are being built in proximity to one another but are not yet fully connected. Therefore, it is still possible for the ship to be brought in pieces into orbit for final assembly.