I believe it was covered on page one, but in short, the rim of the saucer and secondary hull are thicker around the sensor strip, the deflector is a totally different shape, kinda like puckered lips, and the secondary hull is taller and shorter. I believe the pylons are different, too.

Now, someone lock a tractor beam on this thread :p!
Baz! Great idea. Are you going to simply chop up some of your completed ships, or will you be knocking out some nacelle and hull-element designs exclusive to the pack?