Sample:    
   Fifth-letter change (6 letters)
   
   When Felix gets a whiff of ONE,
   
   You see him jump and run,
   But still, before the day is through,
   He's curled up for a TWO.
   
   Answer: ONE is "catnip." By changing the fifth letter, you get TWO, or "catnap."

   1. Beheadment (6, 5 letters). A word becomes a new one when its first letter is removed (example: factor, actor).
   They followed the stream as it FIRST through the wood,
   Staying as much in the shade as they could.
   A splash, and a swimmer then came into sight.
   "He's SECOND!" one Girl Scout cried out in delight.

   2. Curtailment (7, 6). A word becomes a new one when its last letter is removed (example: aspiring, aspirin).
   "Cleopatra," if it's true
   that opposites attract,
   Could be called the ONE of TWO
   By known historic fact.

   3. Deletion (9, 8). A word becomes a new one when an interior letter is removed (example: simile, smile).
So the jury finally TWO that czar of crime?

And a jury trial is ONE? High bloody time.

4. Deletion (5, 4).
   One year fat, one year lean.
   
   Never anywhere between.
   
   Diet changes every day;
   
   Either ONE or TWO, I say.

   Answers:

1. Snaked, naked.
2. Antonym, Antony.
3. Indicated, indicted.
4. Feast, fast.