were any actors from the first film replaced while making the sequel?
Yes, Crispin Glover.
Back to the Future Part II
Crispin Glover was asked to reprise the role of George McFly. 
who played marty in back to the future Michael J. Fox what was the premise of the film? Marty McFly, a teenager accidentally sent back in time to 1955 in a time-traveling DeLorean automobile built by his eccentric scientist friend Doctor Emmett "Doc" Brown. what kind of cultural impact did the film have? "Back to the Future" has influenced or been referenced in a variety of media, including: "American Dad". does it have any sequels? Yes, "Back to the Future Part II". does it have same character mentioned above? Yes. was it played by same actor? Yes.
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Back to the Future Part II [SEP] Replacement of Crispin Glover
Crispin Glover was asked to reprise the role of George McFly. He expressed interest, but could not come to an agreement with the producers regarding his salary. He stated in a 1992 interview on "The Howard Stern Show" that the producers' highest offer was $125,000, less than half of what the other returning cast members were offered. Gale has since asserted that Glover's demands were excessive for an actor of his professional stature at that time. In an interview on the "Opie and Anthony" show in 2013, Glover stated that his primary reason for not doing "Part II" was a philosophical disagreement on the film's message; Glover felt the story rewarded the characters with financial gain, such as Marty's truck, rather than love.