honestly the old one is kind of ugly and isn't going to have much resale value. I would just "rip" it out... literally, with no regard to saving it... Can the insert be removed from the mantle beforehand? there should be a gas shut off valve somewhere behind it. just make sure you close that valve before disconnecting the gas line from the insert. Don't be scared if you hear a little hiss when you first break the connection in the line as there will still be a small amount of gas in there. Nothing to worry about tho. You can probably bet on having to repair the dry wall around it to some level of extent.
hello and welcome....arbutus is right in telling you what your first step should be. sooooo do you live in a mansion because that is the only place that i have ever seen something that huge.heheheheh. maybe you could knock out a wall and just make the room bigger to fit the fire place. it actually sound like a very neat feature to have in a home. might be a silly question but can you list the materials that your piece is made out of. it will make a huge difference on how to remove it and what your tools of choice should be.
When you start this job, keep in mind that in order to disconnect the vent at the back' you will have to break in one side to get behind it and these things come with two upside down "V" out of metal on top right at the front. They are there to make sure the framing wood is kept away. Sometimes they are screwed in from inside or they may be just slipped into notches. If you can't clear them from inside you will need to remove the first row of tile above the unit. Hopefully this thing has just been covered with drywall.