Many people use “downgrade” instead of “denigrate” to mean “defame, slander.” “Downgrade” is entirely different in meaning. When something is downgraded, it is lowered in grade (usually made worse), not just considered worse. “When the president of the company fled to Rio with $15 million, its bonds were downgraded to junk bond status.” “Degrade” is much more flexible in meaning. It can mean to lower in status or rank (like “downgrade”) or to corrupt or make contemptible; but it always has to do with actual reduction in value rather than mere insult, like “denigrate.” Most of the time when people use “downgrade” they would be better off instead using “insult,” “belittle,” or “sneer at.”
While we’re at it, let’s distinguish between “deprecate,” meaning “disapprove,” and “depreciate,” which, like “downgrade,” is not a mere matter of approval or opinion but signifies an actual lowering of value.