Officially with some airlines your ticket will be repriced at the full fare rate even after the fact if this is determined, and any associated FF account may be debited for the miles and/or frozen. While sometimes you might be able to "get away without anyone noticing" (such as Justice Scalia apparently did recently) it would be in your best interest to not say anything about it, just in case. That might be an extreme example, but it does happen occasionally.

Or, pick an airline that doesn't mind. There are some out there that really care, some that look the other way, and some that have official policies that you're welcome to do this.