Yesterday i’ve paid 15 bucks in an used copy of Halo Reach. And yesterday I unlocked the achievement Cross-Mapin’. But this achievement is from the Noble DLC pack, so the Noble Pack is for free now or the Noble Pack came in my disk?
Not all of the “new” achievements are specifically tied to the maps. Only some of them require specific maps/gametypes.
Everyone has their achievement list expanded regardless of whether or not you bought the map pack and some of them achievements don’t require a specific map. So, you can get them.