The expansion pass will upgrade for free. However, if you bought a physical version of the base game, you will have to purchase the next generation version.
Your characters will be on Xbox one, either way, as long as you use the same xbox live ID.
http://www.destinythegame.com/digital-upgrade
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so even if i bought the pass on 360 i only get the dlc free on the 1 not the game? i still have to buy the full game again for the one? wow thanks bungie for your false advertising. con artists of the -blam!-ing year