What is it called when you make a different word by changing one letter of another word?

That's called a mutation, specifically a single-letter mutation or one-letter mutation.

In the context of wordplay, it's often referred to as a jumble or a word puzzle.

Here are some other related terms:

* Anagram: A word or phrase formed by rearranging the letters of another word or phrase.

* Spoonerism: An unintentional transposition of sounds in a word or phrase, often creating a humorous effect.

* Typo: A misspelling caused by an error in typing.

Let me know if you'd like some examples of single-letter mutations!