OR Steve's trip back altered the future. Instead of ending up with Peggy, he ends up with Tony, Civil War doesn't happen. But, it's the same timeline. So the Avengers who went back in time, feel that the past has changed before they "snap" back into the present. So they still have their memories of the old past, but interlaid with the changed past. Over time, they forget the old past. But sometimes, Steve worries that Tony lost Morgan and that he's not enough to compensate for the loss.
Bonus points if they somehow gain a daughter like Morgan.
Re: AU - Steve's trip back in time resulted in another universe
OR Steve's trip back altered the future. Instead of ending up with Peggy, he ends up with Tony, Civil War doesn't happen. But, it's the same timeline. So the Avengers who went back in time, feel that the past has changed before they "snap" back into the present. So they still have their memories of the old past, but interlaid with the changed past. Over time, they forget the old past. But sometimes, Steve worries that Tony lost Morgan and that he's not enough to compensate for the loss.
Bonus points if they somehow gain a daughter like Morgan.