It is also unclear if this will apply to Exotic gear, as well- it will probably not, given the general Rarity and utility of Exotic gear that Bungie probably wants to maintain.
Essentially, it sounds like you will be using other weapons to boost your existing ones’ stats- so there will definitely be a cost.
Destiny 2 is due out on PS4 and Xbox One in September, and then a month later on PC as well.
Of course, it seems that there will be limits to this, so that the balance and gear economy of the game is not thrown off.