I don't know about "extra support pylons". It looks to me like the whole crossbar was replaced with two longer sections going down to the secondary hull. The saucer extensions that held the crossbar would also have been reconfigured - possible parts replaced? The nacelles have changes too. It may be just a paint job, but the glowing blue parts on the outer sides have been replaced by a red Federation stripe. You'd assume the glowy bit had some practical function, so the fact it's gone would indicate (in-world) the whole thing has been replaced.The only real change besides the bolt on parts was that they relocated this part to the secondary hull. But other than that, the NX-01 was unchanged, besides the secondary hull and extra support pylons.