Sea-change or seachange is a poetic or informal term meaning a gradual transformation in which the form is retained but the substance is replaced, as with petrification. The expression is Shakespeare's, taken from the song in The Tempest, when Ariel sings:
Full fathom five thy father lies;Per Wikipedia. Image from Jon Estward's Flickr.
Of his bones are coral made;
Those are pearls that were his eyes:
Nothing of him that doth fade
But doth suffer a sea-change
Into something rich and strange.