A 'good' change in a word. A metaplasm is when you mess around with a word for artistic reasons. If you do it by mistake (or at least, if your mistake is not an improvement), it's a barbarism.
There are many different types of metaplasm, depending on where in the word the change is made, and how it is made. Here's a list; it is probably not complete. Let me know if you have any additions.
Metaplasm by addition:
Diastole: lengthening a syllable.
Ecthlipsis: adding a consonant, perhaps with an accompaning vowel.
Epenthesis: insertion of a letter or a sound
Paragoge: addition of a letter or syllable to the end of a word.
Prothesis or Prosthesis: adding letters to the beginning of a word.
Diaeresis: separation of one syllable into two.
Metaplasm by subtraction or omission:
Aphaeresis or Apheresis: cutting off the beginning of a word.
Apocope: cutting off the end of a word.
Ellipsis: any cutting of a word can be refered top as ellipsis.
Synaloepha: the blending of two successive vowels of adjacent syllables into one syllable.
Synaeresis: two syllables contracted into one.
Syncope: removing letters or syllables from the center of a word.
Systole: shortening a long syllable.
Metaplasm by transposition:
Metathesis: changing the order of letters or syllables in a word.
Metaplasm by substitution:
Antisthecon: subsituting one letter or syllable for another.