In the years between Back to the Future and Back to the Future II, Michael J. Fox visibly aged and the actress playing his girlfriend was replaced by Elizabeth Shue. Thus, to show the closing scene from the first film as the opening of the second, it was necessary to completely reshoot it. Dig?
Anyway, as you can see above, someone has made a side-by-side comparison of the sequence so you can now needlessly critique how well Robert Zemeckis and crew recreated the scene. The audio is layered together as well, and for some reason hearing a duet of Docs saying, in unison, "Roads? Where we're going, we don't need... roads," is utterly hypnotizing:
Luckily, the rest of the film holds up to intense scrutiny, and this is the only slight flaw.