The voice and music recording for "Journey Back to Oz" was actually done a decade earlier than the film's release, in 1964. The producers had to wait several years while sufficient funding was obtained to produce the animated sequences.
According to the book "Creating the Filmation Generation", Danny Thomas voice recording sessions were of such bad quality that they were almost completely unusable, except for his musical number. Larry Storch was brought in to rerecord the Tin Man's dialogue, imitation Thomas, who never knew his voice had been replaced.
Peter Lawford was the original voice of the Scarecrow, but his lines were rerecorded by Mickey Rooney. There were two soundtrack records released, one of which was a Japanese bootleg which still featured Lawford's singing voice instead of Rooney, as well as two songs that were changed or cut from the picture.
The ABC television version featured new wraparound segments featuring Bill Cosby as the Wizard of Oz, standing in a hot air balloon with two children playing munchkins named Sprig and Twig. Cosby was only contracted for two uses, and so when SFM Distribution got the syndication rights, his segments were replaced with new material featuring Milton Berle.