How do I remove an old ceiling junction box to install one rated for a fan?
If you have determined that the existing box is not sufficient to support a fan or you want to make sure that a box is designed for a ceiling fan, it would not be difficult.  After opening the breaker to the box in question.

Remove the old box.  You may have to cut some of the drywall away to remove the old box and to install the support bracket of the new ceiling fan box.
Once the old box is removed, use a ceiling fan mount like this and attach to two joists.
Re-patch the sheetrock and mount the ceiling fan.