At the end of the original Back to the Future, Marty returns home, finds that his future has changed for the better, and that he can spend the rest of the day hanging out with his girlfriend Jennifer. However, this moment of bliss is short lived as Doc Brown enters out of nowhere in the DeLorean time machine. He tells Marty and Jennifer that they need to come with him to the future to help fix the future of their kids! The DeLorean begins to take flight and heads off into the future as the credits roll.
Originally this ending was not intended to set up a sequel but instead just one last joke. However after the box office success of the film two sequels developed. Robert Zemeckis and Bob Gale (the director and writer) have often said that they would not have had Jennifer in the car because it adds another character that needs to tag along and another person that knows about the time machine (this is why she was almost immediately knocked unconscious in part two). Also they never wanted to go into the future because they worried that once the time comes (which is actually in three more years) their future would be out of date and wrong.
However the audience saw this one last joke as a sign of the next adventure so the production team had to include it in the next film. However there were many reasons why they had to reshoot it. For one part one came out in 1985 and part two came out in 1989 and within these four years the main actors (Michael J. Fox and Christopher Lloyd) had visibly aged. They also had to reshoot it because Jennifer had to be re-casted. Claudia Wells, the original Jennifer, had a baby and took a break from acting within the four years so Elizabeth Shue took the part. It would look jarring to the audience if Wells was in one scene and then was replaced by Shue in the next. They also could not skip including the scene entirely because a big plot point of part two was that Biff had to see the DeLorean take off from 1985.
Overall they did a great job recreating it. They went to the same location, got the same props, and obtained the same costumes. They even reused footage that the actors were not in (like the shot of the DeLorean entering and hitting the trash cans). However there are some noticeable small differences. For one the lighting. It was a much sunnier day in part two when they shot and that is clear when Marty first sees Jennifer. There are also small details in the background that many might not notice so it was probably not in the forefront of the mind of the production team. These include the cars in the street behind them and the bushes and flowers around Marty’s house. Overall the biggest difference I could see was with line readings. There are instances where I think the actors thought they paused longer or shook their head a certain line so they overdid it the second time around. Shue also overacts especially with her eyes when Doc tells them about going to the future. However overall I think the production team did a great job of recreating these scenes and unless people watch them side-by-side I bet nobody could easily tell the difference.