fan installation and ceiling boxes

hey everybody. so right now im looking to install a cieling fan in a room that previously just had a simple 2 lightbulb fixture in it. so ive taken down the lighting fixture and found a plastic cieling box that appears to be nailed or riveted to a sheet of aluminum that seems connected to a piece of 2x4 that has a significant amount of play in it...???? anyways i know that i have to replace this with a fan rated box but i cant for the life of me figure out how to get this old box out. as of right now the drywall is sitting snugly around the box and i was only able to see inside a little bit to observe what it appears to be mounted to. i tried tugging on it and tapping it out and such as several books i have read suggested but it wont come out. does anybody have any idea how i need to go about getting this out?

any help is much appreciated!

 

Yep, destroy it. Slip a pry bar between the box and the joist to which it is attached, push up on the loose end and pry it out. If you break the box, so what, you will be replacing it anyway. Now as for your replacement, be sure to use a fan rated box, and you may even be able to use the spanner bar type that are installed between the joists from below, so you don't need access to the top.