Basically, game devs can add new achievements for PS games with any free update or paid DLC. They are kept separate from the "base" game's achievements so they don't break the trophy stuff, but they can be added. Devs can also sometimes reduce the requirements needed to complete achievements that were already in the game, though I assume that may need approval by Sony. I don't believe they can entirely remove an achievement. But since we are talking about adding new achievements, the answer is that yes, they can do so.