I’m guessing all the advanced scenarios introing TW level gear, infantry, and vehicles have been cut. I assume the construction rules remain intact but who knows
The GoAC rulebook does contain construction rules, but my sense is that they're pared down. (I haven't taken a spin through the older box set rulebook in awhile, and don't have enough spare time for a close comparison.)
There are scenarios which use the box contents in both rulebooks. However, GoAC's book does not include infantry or vehicles.
Ten seconds on the Beginner Box rulebook: it's basic by intent, and doesn't include many of the rules that longtime players here know like the back of their hand. As with the Beginner Box as a whole, the intent of the rulebook was to give brand new players a taste of the game: moving Mechs around a map, figuring out ranges, blasting their friends into chunky salsa.