I'm quite sure that there will be a lot of tweeking.
No story that has been referenced so far in the MCU was an adaption.
And if there has to be a bit of fan (fic) speculation, here is my part:
Bucky tries to be a hero at the beginning as Zemo makes his play. Something goes awefully wrong. Tony finds out the WS killed his parents and goes after him.
Cap stops Tony from going after Bucky. Long discussions follow. Things escalte. Tony and Steve are at odds, but must team up to take Zemo down. Cap is forced to go underground at the end, maybe even out of commision after a run in with Cross Bones. But only after he had a moral victory.
At the end Tony promises to make things right and realizes Bucky is not a bad guy after all and gives him the shield to be the next Cap. Maybe he and Bucky both blame themselves that Steve is out of the picture.
The movie ends with Tony as more of an advisor and political figure for phase 3. Thus he can be referenced more or less all through phase 3, but he is not suiting up or building anther house party because of what happened in Age of Ultron.
Avengers Infinity War part 1 sees mostly the heroes of phase 3 fighting a losing battle, after Bucky Cap had a few cameos in the other phase 3 movies. And at the end of Infinity War 1, as all seems lost we see the return of Thor, Cap and Iron Man to turn the tide in part 2.