Kurt Russel would be a great choice for John (Carousel of Progress)
Ideally, I would hope that Disney would cast a working voice actor for the role, but if they went the celebrity actor route, Kurt Russell would have my vote.
Rex Allen and Jean Shepherd both shared qualities of having very rich, warm voices and being active contemporaries of Walt Disney era media. Rex Allen and Jean Shepherd grew up through the most of the events of the original ride (born in 1920 and 1921 respectively).
Kurt Russell has a rich, warm voice. Russell was a contemporary of Walt Disney, being the one of the subjects of Disney’s last writings. Russell grew up and lived through the coming events of the updated ride (born in 1951).
Russel fits in well the shared qualities of Allen and Shepherd and I think would fit well as a new voice actor for John.