https://www.polygon.com/e3/2018/6/12/17456390/destiny-2-year-one-legendaries-forsaken-bungie-redrix-claymore

At E3 2018, Bungie confirmed that players will be able to bring their current legendaries into the Destiny 2: Forsaken, the game’s new expansion this September. While we know that the grind will still be worthwhile this summer, Cotton didn’t explain how legendaries will carry over in his interview with PC Gamer.

“You’ll be able to bring your Year One stuff forward with you,” Destiny 2 game director Steve Cotton told PC Gamer. “If you own Forsaken you’ll be able to bring it up to the same [level].”