Old Ceiling Junction Box Removal
Jim Stewart's comment is pretty much right on.

That is not a nipple it is what is called a "hickey" by the National Electrical Code. It serves to attach the box to a hanger bar or other spanning device.

It looks like you may have a pretty stout box already and may not need the fan box.

I usually look for a ceiling hugger style fan and toggle bolt it directly to the drywall/plaster. This eliminates the need for fan box.

Good luck!