The character of Gopher (not in the book, but at your service) was originally included to replace the original A.A. Milne character, Piglet. The studio eventually reinstated Piglet in the second featurette, Troppo vento per Winny Puh (1968).
Tigger's trademark "Hoo-hoo-hoo-hoo!" laugh by was ad-libbed by Paul Winchell as well as his famous "TTFN: Ta-Ta For Now!" line, at the suggestion of his wife.
When Walt Disney acquired the rights to the Winnie the Pooh characters and stories in 1961 he learned the stories and characters were not as familiar to the Americans as they were to the British. So he planned to make Pooh much more popular to allow and build up an American following.