I’m not familiar with either The Doris Day Show or The Jack Benny Show other than having seen a few episodes of each, but I would say that a wholesale format change would not be a canon error, it would be a change of canon.
The use of the word canon to describe acknowledged works of fiction is somewhat problematic. In an older and more formal sense, canon refers to religious works held to be authoritative as scripture within a given community. Thus, Judaism, Roman Catholicism, and mainstream Protestantism have different but overlapping canon, which have been reinforced by centuries of ecclesiastical authority.
With fiction, it gets a little messier. Sometimes it’s easy, as with Sherlock Holmes: anything written by Arthur Conan Doyle is canon; anything not, isn’t. But with a corporate or collaboritive work, things get messy. In the Star Wars universe, the movies are considered canon. Works in the “Extended Universe” (e.g., novels) are considered canon unless they are later contradicted by the movies. (Splinter of the Mind’s Eye, one of the most popular SW novels, is an example of something that has been de-canonized.) In Star Trek, films and various live-action TV series are canon, books are not, and a single episode of the animated series is. With Tolkien’s Middle-earth, some people would include anything fished out of the professor’s wastebasket for posthumous publication to be canon, while sensible people such as myself recognize only The Hobbit and The Lord of the Rings, with LOTR taking precedence in any case in which they contradict each other.
Which brings up one of my favorite examples of a ret-con. When Tolkien wrote The Hobbit, Gollum was a strange creature of undetermined origin who happened to own a ring of invisibility. In The Lord of the Rings, he became a Hobbit (or Hobbit-like creature) who was completely ensnared and twisted by an artifact of ancient evil. Realizing that the depiction of Gollum and his relation to the ring in the first book was incompatible with his conception of Smeagol and the Ring in the sequel, Tolkien came up with a brilliantly simple explanation.
The Hobbit, you see, was taken from Bilbo’s own account of his adventures, before anyone knew of their true significance. So he did a little whitewashing. The Lord of the Rings, however, drew on later texts that were written after Bilbo had revealed the truth, which is related in the prologue.
For subsequent editions of The Hobbit, Tolkien rewrote encounter with Gollum and the ring to more closely mesh with LOTR.