Expansion does not mean replacement... Expansion would imply that something is being ADDED to the game, or another direction (hence the term expansion). If something is deemed obsolete with a viable substitution, it would be a replacement (upgrade would imply there is an improvement of sorts, but this is subjective). So I think to avoid a lot of the confusion that we've seen over the last couple DLC expansion packs, Bungie could have just renamed them replacement packs.