How much force does an 3 pound hammer hit with? Just 3 pounds, right? Just place the hammer atop the nail and the nail drives in from the fearful force of 3 pounds? No, that's not how hammers work, hammers involve a dynamic force that is much larger.
That's not how fans work either. Fans are mostly dynamic force because of their spinning and vibration. It would be 16 pounds if you never started the fan; likewise the hook on my tool rack only has to contend with 3 pounds.
If you use a non-fan-rated box to hold up a fan, the fasteners will walk themselves out from the vibration, and you'll drop a fan on somebody's head. If that's a welded steel box, it could even crack the welds. (drawn steel boxes won't have that problem, at least).
So you'll need to do a box-ectomy, and the trick is to confine the drywall damage to what will be concealed by the fan shroud.
Tape the 2 wires together (obviously, do this with the breaker off). Since it's conduit, you'll have to find the other end of that conduit and pull back the red and white wire so it's just an inch inside the pipe, then unbolt the conduit (the castellations on that wacky nut are to give you a place to put a screwdriver so you can bap it to make it rotate; remember lefty loosey.)
You have to be careful positioning the new fan-rated box so a knockout on the box will align with the conduit pipe; it's probably EMT and hard to move. There's no benefit to removing the fitting from the end of the pipe unless you need the clearance to work. It's only an inch of clearance anyway. The fitting has to go back on the pipe before the box is fitted because you must tighten it (for grounding; that's the ground).
Then go to the box from which you pulled the wires back, and push them back toward this box. Pushing usually works if there's only one 90-degree-bend in the pipe. Or reach into the pipe with a narrow needle-nose and grab them. This is why you bundled them together: so you could grab them now.
