As previously announced, Destiny 2 will be leaving Blizzard's BattleNet platform and transferring over to Steam on October 1st. Once this happens, Destiny 2 will also be adapting a free-to-play model with optional expansions, introducing a whole bunch of sweeping changes, as well as adding the long-awaited Shadowkeep expansion pack.

In order to have your BattleNet characters survive the move to Steam you'll need to link your two accounts over at the Destiny 2 website. Once you've done this successfully, your player account, expansions, currency, annual pass and that sort of stuff will be fully available once Destiny 2 launches on Steam. 

Similarly, if you're planning on playing Destiny 2 on multiple platforms, you might also want to sign up for the upcoming Cross Platform Save service. You will only be able to designate one account as your primary one, so do make sure to read the full FAQ before you make any sort of mistake.

As for what exactly will happen if you haven't linked your account before October 1st, I'm not exactly certain, but I would highly advise against trying to find out. Destiny 2 is moving away from BattleNet in its entirety, so all of that player data will likely be lost in the process. Bungie's wording in the FAQ isn't exactly positive either, so I'd just bite the bullet and go through the process as soon as possible. After all, two minutes of annoyance is certainly much better than losing your entire account!