I came across a tutorial for the spark plug change on the 5.4 3V here, and they said they needed a 9/16" socket for the plugs.

I checked a handful of mine last night to see if they were loose, as I've heard that can be one cause of the ticking these engines exhibit. They definitely needed a typical 5/8" socket.

Can I take that as a sign that the plugs have been changed at some point, or were there different plugs in different model years? Mine is an '05.

 

I know the Motorcrafts on mine took the weird 9/16" socket and Champions take a standard spark plug socket. So there's a good chance you have Champions or some other aftermarket plug in there. Champions shouldn't break on you so you should be good to replace them.