The "Parody" defense is meant to cover actual rewrites of songs where the meaning is intentionally changed and usually where a great deal of the lyrics have been re-written. It is not a parody to alter one or two words.

Personnaly I usually find that any changes to a song are usually ones I could have made but discarded because I'm afraid of overworking a lyric (or poem). Not that I have anything in circulation greater than in my own house! ;)