When you switch corresponding letters or segments of words, it is called a spoonerism.

A popular example is "nucking futs." The word spoonerism comes from Reverend William Archibald Spooner, a famous lecturer who reportedly did this a lot.

If a spoonerism is switching part of the beginning of a word, a "kniferism" would switch part of the middle of a word, and a "forkerism" would be for the end of a word. An example would be "the Duck and Doochess of Windsor" and "President Hoobert Heever". These aren't as common, and are arguably still just spoonerisms.
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