A type of metaplasm in which a letter, syllable, or sound is substituted for another. It is also spelled antistoechon or antistoecon, although antisthecon seems to be the most common. These are supercool, because they are often used in puns. (And what could be cooler than a pun?)
Examples include:
A pun is its own reword.
Punsters deserve to be drawn and quoted.
If puns are outlawed, only outlaws will have puns!
Of course, not only puns involve the use of antisthecon. You can also force rhymes in song or prose by substituting letters:
The very day I purchased it,
I christened my guitar
As my monophonic symphony,
Six string orchestrar.
-- Six String Orchestra by Harry Chapin.
And from William Shakespeare's Troilus and Cressida,
Troilus: Better at home, if 'would I might' were 'may.'
But to the sport abroad. Are you bound thither?
Aeneas: In all swift haste.
Troilus: Come, go we then togethir.