It doesn't appear to be attached to a spanner bar unless it is molded in. Note no screws or bolts through the base to attach it to a bar. Take needle nose pliers and pry the edges inward. The box will break in pieces until you can see what is up there. If you have a support member, you're good to go with several types of fan rated boxes. We won't know until you do the demo. Post a picture once you have removed the box.

Edit: I see the bar and can make out the rivets you indicate, so it is attached to a bar. Demo is the same. Once the box is removed, the bar can most likely be pushed up until it releases from the sides. Still need that picture, though.

 

Turns out that wasn't a bar just insulation. It's attached to the joist on the side.

 

Just take a "beater" screwdriver and start whacking! Or anything you can use to hit it with a hammer. Be careful of the wires. Once you get the box broken, you should be able to pry the box loose from the joist. Worse case, you have to use a sawz-all to cut the bracket.

 

Here is the best box I have found for you situation:

Southwire Smart Box Adjustable Depth 75 lbs. Light Fixture Support/50 lbs. Ceiling Fan Support MSBFAN - The Home Depot

Madison Electric Products Smart Box Adjustable Depth Light Fixture and Ceiling Fan Support is the best solution for old work and retrofit applications.

www.homedepot.com

 

Then you are golden. The box RJ showed is absolutely the best angle attachment box for fans.