-Although metanoia is usually referring to a "spiritual conversion", its definition can be broadened to be just a "change of heart" (Merriam-Webster). Basically, when taken out of its original religious context, metanoia is when one's opinion on something or something shifts. However, when put back into its original context, it can either refer to one turning towards or away from religion, or at it was interpreted in the Bible, repentance. In terms of rhetoric, metanoia is when one corrects a recently made statement by either changing its wording or just elaborating on what was said. It should be noted that metanoia, in the rhetorical sense, must happen in the same sentence (or, at least, very close proximity) as the original declaration.
-A rhetorical example of metanoia is the Hippocratic Oath: "To help, or at least do no harm". Here, a general promise ("To help") is made, before quickly being expanded on ("Or at least do no harm").