WoW and Diablo used expansion to introduce new content into their games. So does Destiny. And just like all expansion based games you will be required to progress forward in order to enjoy the content. Which means new armor and new weapons. Even if they do allow you to upgrade the old it will be an endgame material like HoW. Therefore even though you could upgrade your stuff you would STILL have to set it aside and use all the new stuff in order to get to that point.