The Expansions are available for Xbox One to players who purchased them on 360 via the Digital Upgrade program. Search for the Expansions in the Xbox One Marketplace using the same account you used on 360, and you should be able to download them for free.
The Taken King is not part of this program. You'll have to buy a 360 version and a One version to play TTK on both consoles.
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Thanks for the info!!