I bought the Ghost Edition in 2014, which included the expansion pack. I do hope Bungie don't expect us to buy the whole game all over again
If you bought Destiny on release, you may be able to upgrade. Check this article for details.
Anyone who bought Destiny digitally before Jan 15th of 2015 could upgrade for free, but if you purchased after that, you're SOL.
There was a brief window of trade-in at the US game retailer, Gamestop, in September.