Right, it may or may not help, depending on the timing.
And if you do see upgrade inventory via EF, sometimes agents will refuse to process the upgrade if you're already on the waitlist, saying "the computer will do it based on the priority." The "solution" to that is to ask to be removed from the waitlist, then tell the agent (or call back and speak to someone else) you want to upgrade your flight into available "C" upgrade inventory. (Hoping that the computer doesn't process the other folks while this is all happening...)